Handle Alpha channel in Canvas capture

content/renderer/pepper/pepper_media_stream_video_track_host.cc

Index: content/renderer/pepper/pepper_media_stream_video_track_host.cc
diff --git a/content/renderer/pepper/pepper_media_stream_video_track_host.cc b/content/renderer/pepper/pepper_media_stream_video_track_host.cc
index d0e47c2c95d35882ec8e5991f31c97e76e2a3d56..456369346419268e4e56184f13b649f59988ca4b 100644
--- a/content/renderer/pepper/pepper_media_stream_video_track_host.cc
+++ b/content/renderer/pepper/pepper_media_stream_video_track_host.cc
@@ -375,6 +375,11 @@ void PepperMediaStreamVideoTrackHost::OnVideoFrame(
   DCHECK(frame.get());
   // TODO(penghuang): Check |frame->end_of_stream()| and close the track.
   PP_VideoFrame_Format ppformat = ToPpapiFormat(frame->format());
+
+  // Drop alpha channel since we do not support it yet.
+  if (frame->format() == media::PIXEL_FORMAT_YV12A)
+    frame->DropYV12AAlphaChannel();
+
   if (ppformat == PP_VIDEOFRAME_FORMAT_UNKNOWN)
     return;