V4L2SVDA: Add a VP9Accelerator implementation utilizing the V4L2 VP9 frame API.

media/gpu/accelerated_video_decoder.h

Index: media/gpu/accelerated_video_decoder.h
diff --git a/media/gpu/accelerated_video_decoder.h b/media/gpu/accelerated_video_decoder.h
index c9b6c29ac8c502051ca49ca2261746827bad7925..ed3db1a50c9e650b16a06ac7c65ff6f6380039a7 100644
--- a/media/gpu/accelerated_video_decoder.h
+++ b/media/gpu/accelerated_video_decoder.h
@@ -44,6 +44,8 @@ class MEDIA_GPU_EXPORT AcceleratedVideoDecoder {
     kAllocateNewSurfaces,  // Need a new set of surfaces to be allocated.
     kRanOutOfStreamData,   // Need more stream data to proceed.
     kRanOutOfSurfaces,     // Waiting for the client to free up output surfaces.
+    kNeedContextUpdate,    // Waiting for the client to update decoding context
+                           // with data acquired from the accelerator.
   };
 
   // Try to decode more of the stream, returning decoded frames asynchronously.