Add mailbox-based Mojo VideoFrame variant.

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 42261246c5b407b853fd05b18791912a7bf81b87..049c7786898f9c70560a7b49ca83dce0c1c15918 100644
--- a/media/mojo/interfaces/video_decoder.mojom
+++ b/media/mojo/interfaces/video_decoder.mojom
@@ -4,6 +4,7 @@
 
 module media.mojom;
 
+import "gpu/ipc/common/sync_token.mojom";
 import "media/mojo/interfaces/media_types.mojom";
 import "mojo/common/unguessable_token.mojom";
 
@@ -68,6 +69,10 @@ interface VideoDecoder {
 };
 
 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.

 };