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 5b0e5c03a6f4b10ec750c6d4bb36e6b8d9bb83f5..f3e341e64498fbf87e95b1bd47d17f83dd5bb7e1 100644
--- a/media/filters/vpx_video_decoder.h
+++ b/media/filters/vpx_video_decoder.h
@@ -55,6 +55,14 @@ class MEDIA_EXPORT VpxVideoDecoder : public VideoDecoder {
     kError
   };
 
+  // Return values for decoding alpha plane.
+  enum AlphaDecodeStatus {
+    kAlphaPlaneProcessed,  // Alpha plane (if found) was decoded successfully.
+    kNoAlphaPlaneData,  // Alpha plane was found, but decoder did not return any
+                        // data.
+    kAlphaPlaneError  // Fatal error occured when trying to decode alpha plane.
+  };
+
   // Handles (re-)initializing the decoder with a (new) config.
   // Returns true when initialization was successful.
   bool ConfigureDecoder(const VideoDecoderConfig& config);
@@ -74,8 +82,14 @@ 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_;
 
   // |state_| must only be read and written to on |offload_task_runner_| if it