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

ash/host/window_tree_host_factory.cc

-// 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.
-
-#include "ash/host/window_tree_host_factory.h"
-
-#include "ui/aura/window_tree_host.h"
-
-namespace {
-
-class WindowTreeHostFactoryImpl : public ash::WindowTreeHostFactory {
- public:
-  WindowTreeHostFactoryImpl() {}
-
-  // Overridden from WindowTreeHostFactory:
-  virtual aura::WindowTreeHost* CreateWindowTreeHost(
-      const gfx::Rect& initial_bounds) OVERRIDE {
-    return aura::WindowTreeHost::Create(initial_bounds);
-  }
-};
-
-}
-
-namespace ash {
-
-// static
-WindowTreeHostFactory* WindowTreeHostFactory::Create() {
-  return new WindowTreeHostFactoryImpl;
-}
-
-}  // namespace ash