Add an additional VDA::Flush() mode to return all allocated buffers.

content/common/gpu/media/v4l2_slice_video_decode_accelerator.h

Index: content/common/gpu/media/v4l2_slice_video_decode_accelerator.h
diff --git a/content/common/gpu/media/v4l2_slice_video_decode_accelerator.h b/content/common/gpu/media/v4l2_slice_video_decode_accelerator.h
index f917e1a4e3bcc7a3cefb63412bb9fe8c7553d14f..1f0a1a9c5b031c58eca0b271dee3d45a6d37c170 100644
--- a/content/common/gpu/media/v4l2_slice_video_decode_accelerator.h
+++ b/content/common/gpu/media/v4l2_slice_video_decode_accelerator.h
@@ -416,7 +416,8 @@ class CONTENT_EXPORT V4L2SliceVideoDecodeAccelerator
   // Decoder state.
   State state_;
 
-  Config::OutputMode output_mode_;
+  // VDA::Config we were Initialize()d with.
+  Config config_;
 
   // If any of these are true, we are waiting for the device to finish decoding
   // all previously-queued frames, so we can finish the flush/reset/surface