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

ash/host/ash_window_tree_host.cc

+// Copyright 2014 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/ash_window_tree_host.h"
+
+#include "ash/root_window_controller.h"
+#include "ui/aura/window_tree_host.h"
+
+namespace ash {
+
+// static 
+AshWindowTreeHost* AshWindowTreeHost::Get(
+    aura::Window* window) {
+  return internal::GetRootWindowController(window->GetRootWindow())->ash_host();
+}
+
+AshWindowTreeHost::~AshWindowTreeHost() {
+}
+
+}  // namesapce ash