views/mus: Install a mus-friend ContextFactory for tests.

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 ui {
+class ContextFactory;
+class ContextFactoryPrivate;
+}
+
 namespace views {
 
 class ViewsTestHelper;
 class Widget;
 
 class PlatformTestHelper {
  public:
   using Factory = base::Callback<std::unique_ptr<PlatformTestHelper>(void)>;
   PlatformTestHelper() {}
-  virtual ~PlatformTestHelper() {}
+  virtual ~PlatformTestHelper();
 
   static void set_factory(const Factory& factory);
   static std::unique_ptr<PlatformTestHelper> Create();
 
   static void SetIsMus();
   static bool IsMus();
 
   // Called once the ViewsTestHelper has been created, but before SetUp() is
   // called.
   virtual void OnTestHelperCreated(ViewsTestHelper* helper) {}
 
   // Simulate an OS-level destruction of the native window held by |widget|.
   virtual void SimulateNativeDestroy(Widget* widget);
 
+  virtual void InitializeContextFactory(
+      ui::ContextFactory** factory,
+      ui::ContextFactoryPrivate** factory_private);
+
  private:
   DISALLOW_COPY_AND_ASSIGN(PlatformTestHelper);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_TEST_PLATFORM_TEST_HELPER_H_