Adds DesktopWindowTreeHostMus

ui/views/test/platform_test_helper.h

 // 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.
 
 #ifndef UI_VIEWS_TEST_PLATFORM_TEST_HELPER_H_
 #define UI_VIEWS_TEST_PLATFORM_TEST_HELPER_H_
 
 #include <memory>
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
 
 namespace views {
 
+class ViewsTestHelper;
+
 class PlatformTestHelper {
  public:
   using Factory = base::Callback<std::unique_ptr<PlatformTestHelper>(void)>;
   PlatformTestHelper() {}
   virtual ~PlatformTestHelper() {}
 
   static void set_factory(const Factory& factory);
   static std::unique_ptr<PlatformTestHelper> Create();
 
   // Whether we are running under the mus environment. Methods are static so
   // that they can be called before Create().
   static void SetIsMus();
   static bool IsMus();
 
+  // Called once the ViewsTestHelper has been created, but before SetUp() is
+  // called.
+  virtual void OnTestHelperCreated(ViewsTestHelper* helper) {}
+
  private:
   DISALLOW_COPY_AND_ASSIGN(PlatformTestHelper);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_TEST_PLATFORM_TEST_HELPER_H_