Move part of display_compositor's interface to cc/ipc

cc/ipc/mojo_compositor_frame_sink.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 cc.mojom;
+
+import "cc/ipc/compositor_frame.mojom";
+import "cc/ipc/returned_resource.mojom";
+
+// A MojoCompositorFrameSink is an interface for receiving CompositorFrame
+// structs. A CompositorFrame contains the complete output meant for display.
+// Each time a client has a graphical update, and receives an OnBeginFrame, it
+// is responsible for creating a CompositorFrame to update its portion of the
+// screen.
+interface MojoCompositorFrameSink {
+  // After the submitted frame is either drawn for the first time by the display
+  // compositor or discarded, the callback will be called. Clients should use
+  // this acknowledgement to ratelimit frame submissions.
+  // TODO(fsamuel): We should support identifying the CF in the callback.
+  SubmitCompositorFrame(cc.mojom.CompositorFrame frame) => ();
+
+  // Lets the display compositor know that the client wishes to receive the next
+  // BeginFrame event.
+  SetNeedsBeginFrame(bool needs_begin_frame);
+
+  // TODO(fsamuel): ReadbackBitmap API would be useful here.
+};
+
+interface MojoCompositorFrameSinkClient {
+  ReturnResources(array<cc.mojom.ReturnedResource> resources);
+};