Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/stub/stub_window.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/stub/stub_window.h"
 
 #include "base/logging.h"
 #include "ui/platform_window/platform_window_delegate.h"
 
 namespace ui {
@@ skipping 12 matching lines @@
 void StubWindow::Show() {
 }
 
 void StubWindow::Hide() {
 }
 
 void StubWindow::Close() {
   delegate_->OnClosed();
 }
 
+void StubWindow::PrepareForShutdown() {}
+
 void StubWindow::SetBounds(const gfx::Rect& bounds) {
   if (bounds_ == bounds)
     return;
   bounds_ = bounds;
   delegate_->OnBoundsChanged(bounds);
 }
 
 gfx::Rect StubWindow::GetBounds() {
   return bounds_;
 }
@@ skipping 25 matching lines @@
 }
 
 void StubWindow::ConfineCursorToBounds(const gfx::Rect& bounds) {
 }
 
 PlatformImeController* StubWindow::GetPlatformImeController() {
   return nullptr;
 }
 
 }  // namespace ui