media: Implement MojoVideoDecoder::CanReadWithoutStalling().

media/mojo/interfaces/video_decoder.mojom

Index: media/mojo/interfaces/video_decoder.mojom
diff --git a/media/mojo/interfaces/video_decoder.mojom b/media/mojo/interfaces/video_decoder.mojom
index cade79cb74b5e3ff8bee7d55f6ba83a384bca54c..36ae91fd26430098d7d4567019679efd14d5d35e 100644
--- a/media/mojo/interfaces/video_decoder.mojom
+++ b/media/mojo/interfaces/video_decoder.mojom
@@ -75,9 +75,16 @@ interface VideoDecoder {
 };
 
 interface VideoDecoderClient {
-  // Output a decoded frame. Frames must be output in presentation order. If
-  // |release_token| is provided, the client should call OnReleaseMailbox() when
-  // it is finished with the frame.
+  // Output a decoded frame. Frames must be output in presentation order.
+  //
+  // When |can_read_without_stalling| is false, preroll is disabled. This is
+  // necessary if the decoder cannot guarantee that it can output another frame,
+  // for example if output buffers are limited or configuration changes require
+  // the return of all outstanding frames.
+  //
+  // If |release_token| is provided, the client shall call OnReleaseMailbox()
+  // when it is finished using the frame.
   OnVideoFrameDecoded(VideoFrame frame,
+                      bool can_read_without_stalling,
                       mojo.common.mojom.UnguessableToken? release_token);
 };