Introdcue AshWindowTreeHost and move ash/chrome specific code in WTH to ash.

ash/cancel_mode.cc

 // Copyright (c) 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 "ash/cancel_mode.h"
 
 #include "ash/root_window_controller.h"
 #include "ash/shell.h"
 #include "ui/aura/window_event_dispatcher.h"
 
 namespace ash {
 
 void DispatchCancelMode() {
   Shell::RootWindowControllerList controllers(
       Shell::GetAllRootWindowControllers());
   for (Shell::RootWindowControllerList::const_iterator i = controllers.begin();
        i != controllers.end(); ++i) {
-    (*i)->host()->dispatcher()->DispatchCancelModeEvent();
+    (*i)->GetHost()->dispatcher()->DispatchCancelModeEvent();
   }
 }
 
 }  // namespace ash