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

ash/host/window_tree_host_factory.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 ASH_HOST_WINDOW_TREE_HOST_FACTORY_H_
-#define ASH_HOST_WINDOW_TREE_HOST_FACTORY_H_
-
-#include "ash/ash_export.h"
-#include "ui/gfx/rect.h"
-
-namespace aura {
-class WindowTreeHost;
-}
-
-namespace ash {
-
-class ASH_EXPORT WindowTreeHostFactory {
- public:
-  virtual ~WindowTreeHostFactory() {}
-
-  static WindowTreeHostFactory* Create();
-
-  // Creates a new aura::WindowTreeHost. The caller owns the returned value.
-  virtual aura::WindowTreeHost* CreateWindowTreeHost(
-      const gfx::Rect& initial_bounds) = 0;
-
- protected:
-  WindowTreeHostFactory() {}
-};
-
-}  // namespace ash
-
-#endif  // ASH_HOST_WINDOW_TREE_HOST_FACTORY_H_