media: Add support for playback of VP8 Alpha video streams

media/ffmpeg/ffmpeg_common.cc

Index: media/ffmpeg/ffmpeg_common.cc
diff --git a/media/ffmpeg/ffmpeg_common.cc b/media/ffmpeg/ffmpeg_common.cc
index b5e4100b3fb5385d3180914021d2f9c91b82ab20..808f91ab0ddc2c9b81edc841ef9258c33b60ff9b 100644
--- a/media/ffmpeg/ffmpeg_common.cc
+++ b/media/ffmpeg/ffmpeg_common.cc
@@ -387,6 +387,12 @@ void AVStreamToVideoDecoderConfig(
     coded_size = natural_size;
   }
 
+  AVDictionaryEntry* webm_alpha =
+      av_dict_get(stream->metadata, "alpha_mode", NULL, 0);
+  if (webm_alpha && !strcmp(webm_alpha->value, "1")) {
+    format = VideoFrame::YV12A;
+  }
+
   config->Initialize(codec,
                      profile,
                      format,
@@ -499,6 +505,8 @@ VideoFrame::Format PixelFormatToVideoFormat(PixelFormat pixel_format) {
     case PIX_FMT_YUV420P:
     case PIX_FMT_YUVJ420P:
       return VideoFrame::YV12;
+    case PIX_FMT_YUVA420P:
+      return VideoFrame::YV12A;
     default:
       DVLOG(1) << "Unsupported PixelFormat: " << pixel_format;
   }
@@ -511,6 +519,8 @@ PixelFormat VideoFormatToPixelFormat(VideoFrame::Format video_format) {
       return PIX_FMT_YUV422P;
     case VideoFrame::YV12:
       return PIX_FMT_YUV420P;
+    case VideoFrame::YV12A:
+      return PIX_FMT_YUVA420P;
     default:
       DVLOG(1) << "Unsupported VideoFrame::Format: " << video_format;
   }