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/api/video_codecs/video_decoder.h b/webrtc/api/video_codecs/video_decoder.h
new file mode 100644
index 0000000..ba62822
--- /dev/null
+++ b/webrtc/api/video_codecs/video_decoder.h
@@ -0,0 +1,86 @@
+/*
+ *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_API_VIDEO_CODECS_VIDEO_DECODER_H_
+#define WEBRTC_API_VIDEO_CODECS_VIDEO_DECODER_H_
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "webrtc/common_types.h"
+#include "webrtc/typedefs.h"
+#include "webrtc/video_frame.h"
+
+namespace webrtc {
+
+class RTPFragmentationHeader;
+// TODO(pbos): Expose these through a public (root) header or change these APIs.
+struct CodecSpecificInfo;
+class VideoCodec;
+
+class DecodedImageCallback {
+ public:
+  virtual ~DecodedImageCallback() {}
+
+  virtual int32_t Decoded(VideoFrame& decodedImage) = 0;
+  // Provides an alternative interface that allows the decoder to specify the
+  // decode time excluding waiting time for any previous pending frame to
+  // return. This is necessary for breaking positive feedback in the delay
+  // estimation when the decoder has a single output buffer.
+  virtual int32_t Decoded(VideoFrame& decodedImage, int64_t decode_time_ms) {
+    // The default implementation ignores custom decode time value.
+    return Decoded(decodedImage);
+  }
+  // TODO(sakal): Remove other implementations when upstream projects have been
+  // updated.
+  virtual void Decoded(VideoFrame& decodedImage,
+                       rtc::Optional<int32_t> decode_time_ms,
+                       rtc::Optional<uint8_t> qp) {
+    Decoded(decodedImage,
+            decode_time_ms ? static_cast<int32_t>(*decode_time_ms) : -1);
+  }
+
+  virtual int32_t ReceivedDecodedReferenceFrame(const uint64_t pictureId) {
+    return -1;
+  }
+
+  virtual int32_t ReceivedDecodedFrame(const uint64_t pictureId) { return -1; }
+};
+
+class VideoDecoder {
+ public:
+  virtual ~VideoDecoder() {}
+
+  virtual int32_t InitDecode(const VideoCodec* codec_settings,
+                             int32_t number_of_cores) = 0;
+
+  virtual int32_t Decode(const EncodedImage& input_image,
+                         bool missing_frames,
+                         const RTPFragmentationHeader* fragmentation,
+                         const CodecSpecificInfo* codec_specific_info = NULL,
+                         int64_t render_time_ms = -1) = 0;
+
+  virtual int32_t RegisterDecodeCompleteCallback(
+      DecodedImageCallback* callback) = 0;
+
+  virtual int32_t Release() = 0;
+
+  // Returns true if the decoder prefer to decode frames late.
+  // That is, it can not decode infinite number of frames before the decoded
+  // frame is consumed.
+  virtual bool PrefersLateDecoding() const { return true; }
+
+  virtual const char* ImplementationName() const { return "unknown"; }
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_API_VIDEO_CODECS_VIDEO_DECODER_H_