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

ash/display/cursor_window_controller.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/display/cursor_window_controller.h"
 
 #include "ash/display/display_controller.h"
 #include "ash/display/mirror_window_controller.h"
 #include "ash/root_window_controller.h"
 #include "ash/shell.h"
@@ skipping 99 matching lines @@
     UpdateCursorImage();
     UpdateContainer();
   }
 }
 
 void CursorWindowController::UpdateContainer() {
   display_ = Shell::GetScreen()->GetPrimaryDisplay();
   if (is_cursor_compositing_enabled_) {
     SetDisplay(display_);
   } else {
-    aura::WindowTreeHost* mirror_host = Shell::GetInstance()->
-        display_controller()->mirror_window_controller()->host();
-    SetContainer(mirror_host ? mirror_host->window() : NULL);
+    SetContainer(Shell::GetInstance()->
+                 display_controller()->
+                 mirror_window_controller()->
+                 GetWindow());
   }
 }
 
 void CursorWindowController::SetDisplay(const gfx::Display& display) {
   if (!is_cursor_compositing_enabled_)
     return;
 
   display_ = display;
   aura::Window* root_window = Shell::GetInstance()->display_controller()->
       GetRootWindowForDisplayId(display.id());
@@ skipping 118 matching lines @@
   delegate_->SetCursorImage(rotated, display_);
   if (cursor_window_) {
     cursor_window_->SetBounds(gfx::Rect(delegate_->size()));
     cursor_window_->SchedulePaintInRect(
         gfx::Rect(cursor_window_->bounds().size()));
     UpdateLocation();
   }
 }
 
 }  // namespace ash