Add GrabWindowSnapshotAsync tests

ui/aura/window_tree_host.cc

Index: ui/aura/window_tree_host.cc
diff --git a/ui/aura/window_tree_host.cc b/ui/aura/window_tree_host.cc
index 2702f2ff1fd579745273953f08aaddaa8ebeda6c..69ff501e52352a8c9f8d1c0e225ed0af83e798c6 100644
--- a/ui/aura/window_tree_host.cc
+++ b/ui/aura/window_tree_host.cc
@@ -73,12 +73,7 @@ class SimpleRootWindowTransformer : public RootWindowTransformer {
 // 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.
-  // 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() {
@@ -168,6 +163,8 @@ RootWindowHost::RootWindowHost()
     : delegate_(NULL) {
 }
 
+void RootWindowHost::DestroyCompositor() { compositor_.reset(); }
+
 void RootWindowHost::CreateCompositor(
     gfx::AcceleratedWidget accelerated_widget) {
   compositor_.reset(new ui::Compositor(GetAcceleratedWidget()));