media/vpx: Add support for VP9 alpha channel

media/filters/vpx_video_decoder.h

Index: media/filters/vpx_video_decoder.h
diff --git a/media/filters/vpx_video_decoder.h b/media/filters/vpx_video_decoder.h
index c900017184ba491ff69173bdb4a99f8a320098a4..3490ff746b177ca5d88a0519ffacd82e8506b65f 100644
--- a/media/filters/vpx_video_decoder.h
+++ b/media/filters/vpx_video_decoder.h
@@ -55,6 +55,9 @@ class MEDIA_EXPORT VpxVideoDecoder : public VideoDecoder {
     kError
   };
 
+  // Return values for decoding alpha plane.
+  enum AlphaDecodeStatus { kOk, kNoAlphaPlaneData, kError };
+
   // Handles (re-)initializing the decoder with a (new) config.
   // Returns true when initialization was successful.
   bool ConfigureDecoder(const VideoDecoderConfig& config);
@@ -68,8 +71,14 @@ class MEDIA_EXPORT VpxVideoDecoder : public VideoDecoder {
                  scoped_refptr<VideoFrame>* video_frame);
 
   bool CopyVpxImageToVideoFrame(const struct vpx_image* vpx_image,
+                                const struct vpx_image* vpx_image_alpha,
                                 scoped_refptr<VideoFrame>* video_frame);
 
+  AlphaDecodeStatus DecodeAlphaPlane(
+      const struct vpx_image* vpx_image,
+      const struct vpx_image** vpx_image_alpha,
+      const scoped_refptr<DecoderBuffer>& buffer);
+
   base::ThreadChecker thread_checker_;
 
   DecoderState state_;