Support alpha channel recording for VPX in MediaRecorder

content/renderer/media_recorder/video_track_recorder.h

Index: content/renderer/media_recorder/video_track_recorder.h
diff --git a/content/renderer/media_recorder/video_track_recorder.h b/content/renderer/media_recorder/video_track_recorder.h
index 895523d696eaf7e2f1d364762e7df8c8b45209f0..ceebbe1a88cce40b0c41f14de424e55020fe4c18 100644
--- a/content/renderer/media_recorder/video_track_recorder.h
+++ b/content/renderer/media_recorder/video_track_recorder.h
@@ -51,6 +51,7 @@ class CONTENT_EXPORT VideoTrackRecorder
   using OnEncodedVideoCB =
       base::Callback<void(const media::WebmMuxer::VideoParameters& params,
                           std::unique_ptr<std::string> encoded_data,
+                          std::unique_ptr<std::string> encoded_alpha_data,
                           base::TimeTicks capture_timestamp,
                           bool is_key_frame)>;
 
@@ -76,6 +77,8 @@ class CONTENT_EXPORT VideoTrackRecorder
                          const scoped_refptr<media::VideoFrame>& frame,
                          base::TimeTicks capture_time);
 
+  bool CanEncodeAlphaChannelForTesting();

[mcasas, 2017/03/09 21:10:50]

There is no need for this function since
VideoTrackRecorderTest is a friend class, it
can directly access 
encoder_->CanEncodeAlphaChannel()

[emircan, 2017/03/10 00:22:47]

Encoder is forward declared though, so it cannot access the methods. I didn't
think it would be worth moving that whole chunk of info into the h file for
this. WDYT?
Also, I think VideoTrackRecorder is too large now and up for a refactor. I will
make a bug. 

[mcasas, 2017/03/11 01:21:22]

OIC. Well, keep it in mind when splitting up the VTRecorders
(maybe add a TODO() + bug) -- it's just a bit sad 
that we have a friend class VideoTrackRecorderTest 
_and_ a ForTesting() method, we should do one or the 
other.

[emircan, 2017/03/13 05:43:53]

Done.

+
   // Used to check that we are destroyed on the same thread we were created.
   base::ThreadChecker main_render_thread_checker_;