Use a mocked compositor for unit tests

ui/gfx/compositor/compositor_test_support.h

Index: ui/gfx/compositor/compositor_test_support.h
diff --git a/ui/gfx/compositor/compositor_test_support.h b/ui/gfx/compositor/compositor_test_support.h
index de520a07121816c87d6a8ff278beb307a1cbc53a..cd7ea2e2b36144b189a3e625075b170609cb27a4 100644
--- a/ui/gfx/compositor/compositor_test_support.h
+++ b/ui/gfx/compositor/compositor_test_support.h
@@ -10,8 +10,13 @@ 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