Add GrabWindowSnapshotAsync tests

ui/aura/window_tree_host.h

 // Copyright (c) 2012 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_AURA_ROOT_WINDOW_HOST_H_
 #define UI_AURA_ROOT_WINDOW_HOST_H_
 
 #include <vector>
 
 #include "base/message_loop/message_loop.h"
@@ skipping 131 matching lines @@
   // Called when the device scale factor of the root window has chagned.
   virtual void OnDeviceScaleFactorChanged(float device_scale_factor) = 0;
 
   // Stop listening events in preparation for shutdown.
   virtual void PrepareForShutdown() = 0;
 
  protected:
   friend class TestScreen;  // TODO(beng): see if we can remove/consolidate.
 
   RootWindowHost();
+  void DestroyCompositor() { compositor_.reset(); }
 
   void CreateCompositor(gfx::AcceleratedWidget accelerated_widget);
 
   // Returns the location of the RootWindow on native screen.
   virtual gfx::Point GetLocationOnNativeScreen() const = 0;
 
   void NotifyHostResized(const gfx::Size& new_size);
 
   RootWindowHostDelegate* delegate_;
 
  private:
   scoped_ptr<ui::Compositor> compositor_;
 
   scoped_ptr<RootWindowTransformer> transformer_;
 
   DISALLOW_COPY_AND_ASSIGN(RootWindowHost);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_ROOT_WINDOW_HOST_H_