Remove PrepareForShutdown

ui/aura/window_tree_host_mac.mm

 // 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 <Cocoa/Cocoa.h>
 
 #include "ui/aura/window_tree_host_mac.h"
 #include "ui/aura/window_tree_host.h"
 
 namespace aura {
@@ skipping 82 matching lines @@
 }
 
 void WindowTreeHostMac::PostNativeEvent(const base::NativeEvent& event) {
   NOTIMPLEMENTED();
 }
 
 void WindowTreeHostMac::OnDeviceScaleFactorChanged(float device_scale_factor) {
   NOTIMPLEMENTED();
 }
 
-void WindowTreeHostMac::PrepareForShutdown() {
-  NOTIMPLEMENTED();
-}
-
 // static
 WindowTreeHost* WindowTreeHost::Create(const gfx::Rect& bounds) {
   return new WindowTreeHostMac(bounds);
 }
 
 // static
 gfx::Size WindowTreeHost::GetNativeScreenSize() {
   NOTIMPLEMENTED();
   return gfx::Size(1024, 768);
 }
 
 }  // namespace aura