Establish mojo service between Canvas (blink) and SurfaceManager (browser)

third_party/WebKit/public/platform/modules/offscreencanvas/offscreen_canvas_surface_service.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 blink.mojom;
+
+// TODO: discuss if this struct can be merged with cc.mojom.SurfaceId;
+// If so, cc/ipc needs to be added to dependency of WebKit/public.
+struct SurfaceId {
+  uint32 id_namespace;
+  uint32 local_id;
+  uint64 nonce;
+};
+
+struct SurfaceSequence {
+  uint32 id_namespace;
+  uint32 sequence;
+};
+
+interface OffscreenCanvasSurfaceService {
+  [ Sync ]
+  GetSurfaceId() => (SurfaceId surface_id);

[Fady Samuel, 2016/06/02 22:32:23]

This shouldn't need to be sync.

[Justin Novosad, 2016/06/07 19:58:44]

It is sync because the JS API that calls this
(HTMLCanvasElement.transferControlToOffscreen) is synchronous.  We absolutely
need a valid surface ID before that JS method returns.

[Fady Samuel, 2016/06/07 20:03:08]

SurfaceIds don't need to be allocated in the browser although the code is
currently structured that way. Ideally the renderer should be allocating the IDs
within its namespace but that's not what we are doing today. We need to fix
that. However I'm not going to block this CL on that. 

+
+  RequestSurfaceCreation(SurfaceId surface_id);
+
+  Require(SurfaceId surface_id, SurfaceSequence sequence);
+  Satisfy(SurfaceSequence sequence);
+};
+
+