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 c803f692d6db146fab39116bd9fbf771c8b7039b..00cf12186552d59c16be28429998f45aeba01972 100644
--- a/webkit/media/filter_helpers.cc
+++ b/webkit/media/filter_helpers.cc
@@ -36,20 +36,21 @@ static void AddDefaultDecodersToCollection(
     const scoped_refptr<base::MessageLoopProxy>& message_loop,
     media::FilterCollection* filter_collection) {
 
-  scoped_refptr<media::FFmpegVideoDecoder> ffmpeg_video_decoder =
-      new media::FFmpegVideoDecoder(message_loop);
-  filter_collection->GetVideoDecoders()->push_back(ffmpeg_video_decoder);
-
   // TODO(phajdan.jr): Remove ifdefs when libvpx with vp9 support is released
   // (http://crbug.com/174287) .
 #if !defined(MEDIA_DISABLE_LIBVPX)
   const CommandLine* cmd_line = CommandLine::ForCurrentProcess();
-  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);
   }
 #endif  // !defined(MEDIA_DISABLE_LIBVPX)
+
+  scoped_refptr<media::FFmpegVideoDecoder> ffmpeg_video_decoder =
+      new media::FFmpegVideoDecoder(message_loop);
+  filter_collection->GetVideoDecoders()->push_back(ffmpeg_video_decoder);

[scherkus (not reviewing), 2013/04/10 00:22:24]

FYI this means that we'll no longer use FFmpeg's VP8 decoder when one of those
two flags are enabled.

Not something you need to worry about for now ... but one day :)

[vignesh, 2013/04/10 17:59:16]

That's not really true, there are checks in vpx_video_decoder.cc (lines 115-126)
to make sure that it handles the right format of videos when the right flags are
passed. For instance, a VP8 video without alpha will always use FFmpeg's VP8
decoder.

 }
 
 void BuildMediaSourceCollection(