aura: Use PlatformWindow from WindowTreeHost.

mojo/services/native_viewport/native_viewport_win.cc

 // Copyright 2013 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 "mojo/services/native_viewport/native_viewport.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "ui/gfx/rect.h"
 #include "ui/platform_window/platform_window_delegate.h"
 #include "ui/platform_window/win/win_window.h"
@@ skipping 71 matching lines @@
   }
 
   virtual void OnLostCapture() OVERRIDE {
   }
 
   virtual void OnAcceleratedWidgetAvailable(
       gfx::AcceleratedWidget widget) OVERRIDE {
     delegate_->OnAcceleratedWidgetAvailable(widget);
   }
 
+  virtual void OnActivationChanged(bool active) OVERRIDE {}
+
   scoped_ptr<ui::PlatformWindow> platform_window_;
   NativeViewportDelegate* delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeViewportWin);
 };
 
 // static
 scoped_ptr<NativeViewport> NativeViewport::Create(
     shell::Context* context,
     NativeViewportDelegate* delegate) {
   return scoped_ptr<NativeViewport>(new NativeViewportWin(delegate)).Pass();
 }
 
 }  // namespace services
 }  // namespace mojo