Use a mocked compositor for unit tests

ui/gfx/compositor/compositor_test_support.h

 // Copyright (c) 2011 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.
 
 #ifndef UI_GFX_COMPOSITOR_TEST_SUPPORT_H_
 #define UI_GFX_COMPOSITOR_TEST_SUPPORT_H_
 #pragma once
 
 namespace ui {
 
 class CompositorTestSupport {
  public:
+  // Called by programs that want to use a real compositor,
+  // but skip the typical initialization of browser_main.cc.
   static void Initialize();
   static void Terminate();
+
+  // Called by tests that want to use a mocked compositor.
+  static void SetupMockCompositor();
 };
 
 }  // namespace ui
 
 #endif  // UI_GFX_COMPOSITOR_TEST_SUPPORT_H_