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);
 };
 
 ////////////////////////////////////////////////////////////////////////////////
 // RootWindowHost, public:
 
 RootWindowHost::~RootWindowHost() {
-  // 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.

[enne (OOO), 2014/01/03 23:22:40]

Unfortunately, this does have adverse effects if I run the snapshot unit tests
with a real context (as this patch adds).

~RootWindowHostX11() calls XDestroyWindow.  Later, when the compositor gets
destroyed, it flushes the command buffer, which could call
NativeViewGLSurfaceOSMesa::SwapBuffers which calls XGetWindowAttributes on the
destroyed window, getting a BadWindow error.

At least naively it seems like the compositor needs to be destroyed before the
window hierarchy.

-  // 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 RootWindowHost::InitHost() {
   compositor_->SetScaleAndSize(GetDeviceScaleFactorFromDisplay(window()),
                                GetBounds().size());
   window()->Init(ui::LAYER_NOT_DRAWN);
   compositor_->SetRootLayer(window()->layer());
   transformer_.reset(
       new SimpleRootWindowTransformer(window(), gfx::Transform()));
   UpdateRootWindowSize(GetBounds().size());
@@ skipping 88 matching lines @@
   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