Mus: Introduce DisplayCompositorClient mojo interface

cc/ipc/display_compositor.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/surface_id.mojom";
+import "ui/gfx/geometry/mojo/geometry.mojom";
+
+interface DisplayCompositorClient {
+  // Called when a CompositorFrame is submitted to a new SurfaceId for the first
+  // time.

[rjkroege, 2016/10/26 17:48:19]

drive by ask for more wordy docs.

Can you explain more. Any client? Does it have to listen? When it is called?
etc.? Threading rules? How does this fit into the surface flow etc.? I know
you've written some of this in your design docs already so maybe it could be
added here.

+  OnSurfaceCreated(cc.mojom.SurfaceId surface_id,
+                   gfx.mojom.Size frame_size,
+                   float device_scale_factor);
+};