media: Implement MojoVideoDecoder::CanReadWithoutStalling().

media/mojo/interfaces/video_decoder.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module media.mojom;
 
 import "gpu/ipc/common/sync_token.mojom";
 import "media/mojo/interfaces/media_types.mojom";
 import "mojo/common/unguessable_token.mojom";
 
@@ skipping 57 matching lines @@
   Reset() => ();
 
   // Inform the decoder that the client is finished with a frame.
   // |release_sync_token| is a (possibly empty) sync point after which the
   // frame's backing textures will be safe to overwrite.
   OnReleaseMailbox(mojo.common.mojom.UnguessableToken release_token,
                    gpu.mojom.SyncToken release_sync_token);
 };
 
 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);
 };