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";
+
+// The DisplayCompositorClient interface is implemented by the Display
+// Compositor Host, a stable, and privileged peer service to the display
+// compositor. The display compositor host is either the browser process in
+// Chrome or the window server process.
+interface DisplayCompositorClient {
+  // Called when a CompositorFrame is submitted to a new SurfaceId for the first
+  // time.

[rjkroege, 2016/10/26 18:37:03]

When? i.e.: can the DCH rely on this being called immediately on receipt of the
surface? Or can the DC defer such updates under certain circumstances?

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