Add GrabWindowSnapshotAsync tests

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 55 matching lines @@
   const Window* root_window_;
   const gfx::Transform transform_;
 
   DISALLOW_COPY_AND_ASSIGN(SimpleRootWindowTransformer);
 };
 
 ////////////////////////////////////////////////////////////////////////////////
 // WindowTreeHost, public:
 
 WindowTreeHost::~WindowTreeHost() {
-  // TODO(beng): this represents an ordering change. In the old code, the
-  //             compositor was reset before the window hierarchy was destroyed.
-  //             verify that this has no adverse effects.
-  // Make sure to destroy the compositor before terminating so that state is
-  // cleared and we don't hit asserts.
-  compositor_.reset();
+  DCHECK(!compositor_) << "compositor must be destroyed before root window";
 }
 
 void WindowTreeHost::InitHost() {
   window()->Init(ui::LAYER_NOT_DRAWN);
   InitCompositor();
   UpdateRootWindowSize(GetBounds().size());
   Env::GetInstance()->NotifyRootWindowInitialized(delegate_->AsRootWindow());
   window()->Show();
 }
 
@@ skipping 73 matching lines @@
   *point = gfx::ToFlooredPoint(point_3f.AsPointF());
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // WindowTreeHost, protected:
 
 WindowTreeHost::WindowTreeHost()
     : delegate_(NULL) {
 }
 
+void WindowTreeHost::DestroyCompositor() { 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.
   compositor_->SetScaleAndSize(GetDeviceScaleFactorFromDisplay(window()),
                                new_size);
 
   // The layer, and the observers should be notified of the
   // transformed size of the root window.
   UpdateRootWindowSize(new_size);
   delegate_->OnHostResized(new_size);
 }
 
 }  // namespace aura