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..cbc264cadc013fd041fdb07d84fb2b78980ed15b 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.

[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() {