Fix circular dependencies in webrtc_common.

One reason for the circular deps is that common_types.h is a
historical dumping ground for various structs and defines that
are believed to be generally useful. I tried moving things out
that did not appear to be used downstream (StreamCounters,
RtpCounters etc) and moved the things that seemed used
(RtpHeader + supporting structs) to a new file api/rtp_headers.h.
This makes their place in the api more clear while moving out
the things that don't belong in the API in the first place.

I had to extract out typedefs.h from webrtc_common to resolve
another circular dependency. I believe checks includes typedefs,
but common depends on checks.

Bug: webrtc:7745
Change-Id: I725d49616b1ec0cdc8b74be7c078f7a4d46f084b
Reviewed-on: https://webrtc-review.googlesource.com/33001
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21295}
diff --git a/modules/video_coding/BUILD.gn b/modules/video_coding/BUILD.gn
index 4e4ab75..02b0a1b 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -99,6 +99,7 @@
     "..:module_api",
     "..:module_api_public",
     "../..:webrtc_common",
+    "../../:typedefs",
     "../../api:optional",
     "../../api:video_frame_api_i420",
     "../../call:video_stream_api",
@@ -127,6 +128,7 @@
   deps = [
     ":video_coding_utility",
     "../..:webrtc_common",
+    "../../:typedefs",
     "../../test:test_support",
   ]
 }
@@ -169,6 +171,7 @@
   deps = [
     "..:module_api",
     "../..:webrtc_common",
+    "../../:typedefs",
     "../../api:optional",
     "../../api/video_codecs:video_codecs_api",
     "../../common_video",
@@ -239,6 +242,7 @@
   deps = [
     ":video_coding_utility",
     "../..:webrtc_common",
+    "../../:typedefs",
     "../../api:video_frame_api_i420",
     "../../common_video:common_video",
     "../../rtc_base:checks",
@@ -301,6 +305,7 @@
     ":video_coding_utility",
     "..:module_api",
     "../..:webrtc_common",
+    "../../:typedefs",
     "../../api:optional",
     "../../api/video_codecs:video_codecs_api",
     "../../common_video",
@@ -431,6 +436,7 @@
       ":video_coding_utility",
       ":webrtc_vp8",
       "../..:webrtc_common",
+      "../../:typedefs",
       "../../api:video_frame_api_i420",
       "../../api/video_codecs:video_codecs_api",
       "../../common_video:common_video",
@@ -599,6 +605,7 @@
       ":webrtc_vp9",
       "..:module_api",
       "../..:webrtc_common",
+      "../../:typedefs",
       "../../api:video_frame_api",
       "../../api:video_frame_api_i420",
       "../../api/video_codecs:video_codecs_api",