aura: Destroy the compositor before destroying the backing window.

ui/aura/window_tree_host.cc

 // Copyright (c) 2013 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/aura/window_tree_host.h"
 
 #include "ui/aura/env.h"
 #include "ui/aura/root_window.h"
 #include "ui/aura/root_window_transformer.h"
 #include "ui/aura/window.h"
@@ skipping 149 matching lines @@
   *point = gfx::ToFlooredPoint(point_3f.AsPointF());
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // WindowTreeHost, protected:
 
 WindowTreeHost::WindowTreeHost()
     : delegate_(NULL) {
 }
 
-void WindowTreeHost::DestroyCompositor() { compositor_.reset(); }
+void WindowTreeHost::DestroyCompositor() {
+  DCHECK(GetAcceleratedWidget());
+  compositor_.reset();
+}
 
 void WindowTreeHost::CreateCompositor(
     gfx::AcceleratedWidget accelerated_widget) {
   compositor_.reset(new ui::Compositor(GetAcceleratedWidget()));
   DCHECK(compositor_.get());
 }
 
 void WindowTreeHost::NotifyHostResized(const gfx::Size& new_size) {
   // The compositor should have the same size as the native root window host.
   // Get the latest scale from display because it might have been changed.
@@ skipping 11 matching lines @@
 WindowTreeHost* WindowTreeHost::Create(const gfx::Rect& bounds) {
   // This is only hit for tests and ash, right now these aren't an issue so
   // adding the CHECK.
   // TODO(sky): decide if we want a factory.
   CHECK(false);
   return NULL;
 }
 #endif
 
 }  // namespace aura