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