media/vpx: Add support for VP9 alpha channel

media/base/video_frame.h

Index: media/base/video_frame.h
diff --git a/media/base/video_frame.h b/media/base/video_frame.h
index fd5728ca6266062c4f1c1f6fcd4ea0a68faee2e4..09d1ca51b9d0991a985cc8fdd1efa655cabc4dc8 100644
--- a/media/base/video_frame.h
+++ b/media/base/video_frame.h
@@ -194,6 +194,23 @@ class MEDIA_EXPORT VideoFrame : public base::RefCountedThreadSafe<VideoFrame> {
       const gfx::GpuMemoryBufferHandle& v_handle,
       base::TimeDelta timestamp);
 
+  // Wraps external YUVA data of the given parameters with a VideoFrame.
+  // The returned VideoFrame does not own the data passed in.
+  static scoped_refptr<VideoFrame> WrapExternalYuvaData(
+      VideoPixelFormat format,
+      const gfx::Size& coded_size,
+      const gfx::Rect& visible_rect,
+      const gfx::Size& natural_size,
+      int32_t y_stride,
+      int32_t u_stride,
+      int32_t v_stride,
+      int32_t a_stride,
+      uint8_t* y_data,
+      uint8_t* u_data,
+      uint8_t* v_data,
+      uint8_t* a_data,
+      base::TimeDelta timestamp);
+
 #if defined(OS_LINUX)
   // Wraps provided dmabufs
   // (https://www.kernel.org/doc/Documentation/dma-buf-sharing.txt) with a