Reland of Move video_encoder.h and video_decoder.h to /api and create GN targets for them (patchset #1 id:1 of https://codereview.webrtc.org/2794033002/ )

Reason for revert:
Reland with temporary deprecated API to not break chromium and google3.

Original issue's description:
> Revert of Move video_encoder.h and video_decoder.h to /api and create GN targets for them (patchset #8 id:140001 of https://codereview.webrtc.org/2780943003/ )
>
> Reason for revert:
> Suspect of breaking Chrome FYI bots.
>
> See
> https://build.chromium.org/p/chromium.webrtc.fyi/builders/Mac%20Builder/builds/23065
> https://build.chromium.org/p/chromium.webrtc.fyi/builders/Android%20Builder
>
> Example logs:
> ../../content/renderer/media/gpu/rtc_video_encoder_unittest.cc:18:46: fatal error: third_party/webrtc/video_encoder.h: No such file or directory
>  #include "third_party/webrtc/video_encoder.h"
>                                               ^
>
> Original issue's description:
> > Move video_encoder.h and video_decoder.h to /api and create GN targets for them
> >
> > BUG=webrtc:5881
> > # Because PRESUBMIT ignores LINT blacklist for moved files and these
> > # headers have some not easy to resolve issues.
> > NOPRESUBMIT=True
> >
> > Review-Url: https://codereview.webrtc.org/2780943003
> > Cr-Commit-Position: refs/heads/master@{#17511}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/c42f54057050c933008a49d57582577bfb9aed25
>
> TBR=solenberg@webrtc.org,sprang@webrtc.org,ilnik@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:5881
>
> Review-Url: https://codereview.webrtc.org/2794033002
> Cr-Commit-Position: refs/heads/master@{#17514}
> Committed: https://chromium.googlesource.com/external/webrtc/+/716d7ac5c1ed6e392e264b34065800bbf03772b3

TBR=solenberg@webrtc.org,sprang@webrtc.org,guidou@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5881

Review-Url: https://codereview.webrtc.org/2795163002
Cr-Commit-Position: refs/heads/master@{#17537}
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index c6cbfca..9b611bb 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -129,6 +129,7 @@
 
   deps = [
     "../..:webrtc_common",
+    "../../api/video_codecs:video_codecs_api",
     "../../base:rtc_base_approved",
     "../../base:rtc_numerics",
     "../../base:rtc_task_queue",
@@ -224,6 +225,7 @@
   deps = [
     ":video_coding_utility",
     "../..:webrtc_common",
+    "../../api/video_codecs:video_codecs_api",
     "../../base:rtc_base_approved",
     "../../common_video",
     "../../system_wrappers",
@@ -322,6 +324,7 @@
       ":video_coding_utility",
       ":webrtc_vp8",
       "../..:webrtc_common",
+      "../../api/video_codecs:video_codecs_api",
       "../../base:rtc_base_approved",
       "../../common_video:common_video",
       "../../system_wrappers:system_wrappers",
@@ -528,6 +531,7 @@
       ":webrtc_vp9",
       "../..:webrtc_common",
       "../../api:video_frame_api",
+      "../../api/video_codecs:video_codecs_api",
       "../../base:rtc_base",
       "../../base:rtc_base_approved",
       "../../base:rtc_task_queue",
diff --git a/webrtc/modules/video_coding/codecs/test/video_codec_test.h b/webrtc/modules/video_coding/codecs/test/video_codec_test.h
index 65d20ee..9619273 100644
--- a/webrtc/modules/video_coding/codecs/test/video_codec_test.h
+++ b/webrtc/modules/video_coding/codecs/test/video_codec_test.h
@@ -13,12 +13,12 @@
 
 #include <memory>
 
+#include "webrtc/api/video_codecs/video_decoder.h"
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/base/criticalsection.h"
 #include "webrtc/base/event.h"
 #include "webrtc/base/thread_annotations.h"
 #include "webrtc/test/gtest.h"
-#include "webrtc/video_decoder.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {
 
diff --git a/webrtc/modules/video_coding/codecs/vp8/simulcast_rate_allocator.h b/webrtc/modules/video_coding/codecs/vp8/simulcast_rate_allocator.h
index 8e7c792..e558290 100644
--- a/webrtc/modules/video_coding/codecs/vp8/simulcast_rate_allocator.h
+++ b/webrtc/modules/video_coding/codecs/vp8/simulcast_rate_allocator.h
@@ -16,10 +16,10 @@
 #include <map>
 #include <memory>
 
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/common_video/include/video_bitrate_allocator.h"
 #include "webrtc/modules/video_coding/codecs/vp8/temporal_layers.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {
 
diff --git a/webrtc/modules/video_coding/include/video_codec_interface.h b/webrtc/modules/video_coding/include/video_codec_interface.h
index 871cb4b..0bef7da 100644
--- a/webrtc/modules/video_coding/include/video_codec_interface.h
+++ b/webrtc/modules/video_coding/include/video_codec_interface.h
@@ -14,12 +14,12 @@
 #include <vector>
 
 #include "webrtc/api/video/video_frame.h"
+#include "webrtc/api/video_codecs/video_decoder.h"
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/include/module_common_types.h"
 #include "webrtc/modules/video_coding/include/video_error_codes.h"
 #include "webrtc/typedefs.h"
-#include "webrtc/video_decoder.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {
 
diff --git a/webrtc/modules/video_coding/utility/quality_scaler.h b/webrtc/modules/video_coding/utility/quality_scaler.h
index bf81e76..a07b1ea 100644
--- a/webrtc/modules/video_coding/utility/quality_scaler.h
+++ b/webrtc/modules/video_coding/utility/quality_scaler.h
@@ -14,7 +14,7 @@
 #include <utility>
 
 #include "webrtc/common_types.h"
-#include "webrtc/video_encoder.h"
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/base/optional.h"
 #include "webrtc/base/sequenced_task_checker.h"
 #include "webrtc/modules/video_coding/utility/moving_average.h"
diff --git a/webrtc/modules/video_coding/video_codec_initializer_unittest.cc b/webrtc/modules/video_coding/video_codec_initializer_unittest.cc
index d9d1d03..cd269f4 100644
--- a/webrtc/modules/video_coding/video_codec_initializer_unittest.cc
+++ b/webrtc/modules/video_coding/video_codec_initializer_unittest.cc
@@ -8,12 +8,12 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/common_video/include/video_bitrate_allocator.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/video_coding/codecs/vp8/temporal_layers.h"
 #include "webrtc/modules/video_coding/include/video_codec_initializer.h"
 #include "webrtc/test/gtest.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {