media: Add support for playback of VP8 Alpha video streams

webkit/media/filter_helpers.cc

Index: webkit/media/filter_helpers.cc
diff --git a/webkit/media/filter_helpers.cc b/webkit/media/filter_helpers.cc
index 57034129cf66b9f364fecc8f998ede5a21ec7301..5b984b691dc0218228672e78edaeb82a1dcc7624 100644
--- a/webkit/media/filter_helpers.cc
+++ b/webkit/media/filter_helpers.cc
@@ -41,15 +41,17 @@ static void AddDefaultDecodersToCollection(
     filter_collection->GetAudioDecoders()->push_back(opus_audio_decoder);
   }
 
-  scoped_refptr<media::FFmpegVideoDecoder> ffmpeg_video_decoder =
-      new media::FFmpegVideoDecoder(message_loop);
-  filter_collection->GetVideoDecoders()->push_back(ffmpeg_video_decoder);
-
-  if (cmd_line->HasSwitch(switches::kEnableVp9Playback)) {
+  if (cmd_line->HasSwitch(switches::kEnableVp9Playback) ||
+      cmd_line->HasSwitch(switches::kEnableVp8AlphaPlayback)) {
     scoped_refptr<media::VpxVideoDecoder> vpx_video_decoder =
         new media::VpxVideoDecoder(message_loop);
     filter_collection->GetVideoDecoders()->push_back(vpx_video_decoder);
   }
+
+  scoped_refptr<media::FFmpegVideoDecoder> ffmpeg_video_decoder =
+      new media::FFmpegVideoDecoder(message_loop);
+  filter_collection->GetVideoDecoders()->push_back(ffmpeg_video_decoder);
+
 }
 
 bool BuildMediaStreamCollection(