Use a mocked compositor for unit tests

ui/aura/test/test_suite.cc

 // 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.
 
 #include "ui/aura/test/test_suite.h"
 
 #include "base/file_path.h"
 #include "base/path_service.h"
 #include "build/build_config.h"
 #include "ui/base/resource/resource_bundle.h"
@@ skipping 11 matching lines @@
 void AuraTestSuite::Initialize() {
   base::TestSuite::Initialize();
 
   gfx::RegisterPathProvider();
   ui::RegisterPathProvider();
 
   // Force unittests to run using en-US so if we test against string
   // output, it'll pass regardless of the system language.
   ui::ResourceBundle::InitSharedInstance("en-US");
   ui::CompositorTestSupport::Initialize();
+  ui::CompositorTestSupport::SetupMockCompositor();
 }
 
 void AuraTestSuite::Shutdown() {
   ui::CompositorTestSupport::Terminate();
   ui::ResourceBundle::CleanupSharedInstance();
 
   base::TestSuite::Shutdown();
 }
 
 }  // namespace test
 }  // namespace aura