Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/android/platform_window_android.cc

 // Copyright 2015 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 "ui/platform_window/android/platform_window_android.h"
 
 #include <android/input.h>
 #include <android/native_window_jni.h>
 
 #include "base/android/context_utils.h"
@@ skipping 160 matching lines @@
 }
 
 void PlatformWindowAndroid::Hide() {
   // Nothing to do. View is always visible.
 }
 
 void PlatformWindowAndroid::Close() {
   delegate_->OnCloseRequest();
 }
 
+void PlatformWindowAndroid::PrepareForShutdown() {}
+
 void PlatformWindowAndroid::SetBounds(const gfx::Rect& bounds) {
   NOTIMPLEMENTED();
 }
 
 gfx::Rect PlatformWindowAndroid::GetBounds() {
   return gfx::Rect(size_);
 }
 
 void PlatformWindowAndroid::SetTitle(const base::string16& title) {
   NOTIMPLEMENTED();
@@ skipping 33 matching lines @@
 
 void PlatformWindowAndroid::ConfineCursorToBounds(const gfx::Rect& bounds) {
   NOTIMPLEMENTED();
 }
 
 PlatformImeController* PlatformWindowAndroid::GetPlatformImeController() {
   return &platform_ime_controller_;
 }
 
 }  // namespace ui