Add mailbox-based Mojo VideoFrame variant.

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";
 
 // Identifies a GpuCommandBufferStub. MediaGpuChannelManager is responsible
 // for minting |channel_token| objects.
 struct CommandBufferId {
   mojo.common.mojom.UnguessableToken channel_token;
   int32 route_id;
 };
 
@@ skipping 44 matching lines @@
   // the Decode(EOS) callback. (That is, they must flush.)
   Decode(DecoderBuffer buffer) => (DecodeStatus status);
 
   // Reset the decoder. All ongoing Decode() requests must be completed or
   // aborted before executing the callback. This must not be called while there
   // is a pending Initialize() request.
   Reset() => ();
 };
 
 interface VideoDecoderClient {
-  // Output a decoded frame. Frames must be output in presentation order.
-  OnVideoFrameDecoded(VideoFrame frame);
+  // Output a decoded frame. Frames must be output in presentation order. If
+  // the video decoder is still running, the callback will be called when the
+  // client has finished using the video frame. (That is, when it it safe to
+  // reclaim backing resources.)

[xhwang, 2017/01/25 18:25:09]

+jrummell since we need to solve the same problem in Decryptor for shared memory
based VideoFrames. But in the Decryptor case, we'd need a callback to a
callback, which is probably not supported in mojo today.

[sandersd (OOO until July 31), 2017/01/26 23:49:05]

FYI the solution proposed here only works properly for mailboxes given certain
assumptions about the command buffer. If your solution also involves mailboxes
then we should meet to verify that the same restrictions apply.

+  OnVideoFrameDecoded(VideoFrame frame) =>
+      (gpu.mojom.SyncToken? release_sync_token);

[xhwang, 2017/01/25 18:25:09]

What does it mean to call the callback with a null token (since
|release_sync_token| is optional)?

[sandersd (OOO until July 31), 2017/01/26 23:49:05]

Note: I've replaced this mechanism in the current CL.

But the same thing applies in the new code: if the frame was not a
mailbox-backed frame, or the release sync token was never set, then we don't
have a sync point to pass back.

 };