Support alpha channel recording for VPX in MediaRecorder

content/renderer/media_recorder/media_recorder_handler.h

Index: content/renderer/media_recorder/media_recorder_handler.h
diff --git a/content/renderer/media_recorder/media_recorder_handler.h b/content/renderer/media_recorder/media_recorder_handler.h
index 30a763796144e3438f81bd757b765e07b70fc73e..a4014e0d524b16ed92ea4ae2c0a655666d055d6d 100644
--- a/content/renderer/media_recorder/media_recorder_handler.h
+++ b/content/renderer/media_recorder/media_recorder_handler.h
@@ -67,6 +67,7 @@ class CONTENT_EXPORT MediaRecorderHandler final
 
   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,