Introduce DisplayCompositor Mojo API for Mus

components/mus/public/interfaces/display_compositor_host.mojom

Index: components/mus/public/interfaces/display_compositor_host.mojom
diff --git a/components/mus/public/interfaces/display_compositor_host.mojom b/components/mus/public/interfaces/display_compositor_host.mojom
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+// 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 mus.mojom;
+
+import "components/mus/public/interfaces/display_compositor.mojom";
+
+// This is a privileged API to be used only by the Mus Window Server.
+// There is one Display per accelerated widget. This corresponds to a physical
+// display on Chrome OS, and a window on other Chrome platforms.
+interface Display {
+  // Each client (process) has a unique |client_id| and implements the
+  // DisplayClient interface.
+  CreateClient(uint32 client_id, DisplayClient& client);
+};
+
+// This is a privileged API to be used only by the Mus Window Server.
+// There should be only one DisplayCompositor connection.
+interface DisplayCompositor {
+  CreateDisplay(int32 accelerated_widget,
+                Display& display,
+                DisplayClient client);
+};