Support alpha channel recording for VPX in MediaRecorder

content/renderer/media_recorder/media_recorder_handler.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_RENDERER_MEDIA_RECORDER_MEDIA_RECORDER_HANDLER_H_
 #define CONTENT_RENDERER_MEDIA_RECORDER_MEDIA_RECORDER_HANDLER_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
@@ skipping 49 matching lines @@
   bool start(int timeslice) override;
   void stop() override;
   void pause() override;
   void resume() override;
 
  private:
   friend class MediaRecorderHandlerTest;
 
   void OnEncodedVideo(const media::WebmMuxer::VideoParameters& params,
                       std::unique_ptr<std::string> encoded_data,
+                      std::unique_ptr<std::string> encoded_alpha_data,

[mcasas, 2017/03/08 22:38:48]

It's unfortunate that the frame and its alpha must go
together, but if so, could you add some comment here
and in the webm_muxer declaration to state it?

Otherwise I might think that alpha and non-alpha 
could be sent separately and marked in some way.

Also what about s/encoded_alpha_data/encoded_alpha/ ?

[emircan, 2017/03/09 01:45:40]

They are used together in the same function call in webm_muxer so I think it
would make sense to send them together. 
Changed the name. Added a comment.

                       base::TimeTicks timestamp,
                       bool is_key_frame);
   void OnEncodedAudio(const media::AudioParameters& params,
                       std::unique_ptr<std::string> encoded_data,
                       base::TimeTicks timestamp);
   void WriteData(base::StringPiece data);
 
   // Updates |video_tracks_|,|audio_tracks_| and returns true if any changed.
   bool UpdateTracksAndCheckIfChanged();
 
@@ skipping 33 matching lines @@
   // Worker class doing the actual Webm Muxing work.
   std::unique_ptr<media::WebmMuxer> webm_muxer_;
 
   base::WeakPtrFactory<MediaRecorderHandler> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaRecorderHandler);
 };
 
 }  // namespace content
 #endif  // CONTENT_RENDERER_MEDIA_RECORDER_MEDIA_RECORDER_HANDLER_H_