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"
 #include "ui/base/ui_base_paths.h"
+#include "ui/gfx/compositor/compositor.h"
+#include "ui/gfx/compositor/test_compositor.h"
 #include "ui/gfx/gfx_paths.h"
 #include "ui/gfx/gl/gl_implementation.h"
 
 namespace aura {
 namespace test {
 
 AuraTestSuite::AuraTestSuite(int argc, char** argv)
     : TestSuite(argc, argv) {}
 
 void AuraTestSuite::Initialize() {
 #if defined(OS_LINUX)
   gfx::InitializeGLBindings(gfx::kGLImplementationOSMesaGL);
 #endif
   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");
+
+  // Use a mock compositor that noops draws.
+  ui::Compositor::set_compositor_factory_for_testing(
+      ui::TestCompositor::Create);
 }
 
 void AuraTestSuite::Shutdown() {
   ui::ResourceBundle::CleanupSharedInstance();
 
   base::TestSuite::Shutdown();
 }
 
 }  // namespace test
 }  // namespace aura