Fix AshWindowTreeHost shutdown for Ozone X11.

ui/ozone/platform/drm/host/drm_window_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 "ui/ozone/platform/drm/host/drm_window_host.h"
 
 #include "base/bind.h"
 #include "ui/display/display.h"
 #include "ui/events/devices/device_data_manager.h"
 #include "ui/events/event.h"
@@ skipping 56 matching lines @@
 
 void DrmWindowHost::Show() {
 }
 
 void DrmWindowHost::Hide() {
 }
 
 void DrmWindowHost::Close() {
 }
 
+void DrmWindowHost::PrepareForShutdown() {}
+
 void DrmWindowHost::SetBounds(const gfx::Rect& bounds) {
   bounds_ = bounds;
   delegate_->OnBoundsChanged(bounds);
   SendBoundsChange();
 }
 
 gfx::Rect DrmWindowHost::GetBounds() {
   return bounds_;
 }
 
@@ skipping 111 matching lines @@
 void DrmWindowHost::SendBoundsChange() {
   // Update the cursor before the window so that the cursor stays within the
   // window bounds when the window size shrinks.
   cursor_->CommitBoundsChange(widget_, bounds_, GetCursorConfinedBounds());
   sender_->GpuWindowBoundsChanged(widget_, bounds_);
 
   overlay_manager_->ResetCache();
 }
 
 }  // namespace ui