chromeos: makes it possible for windows to outlive WindowTreeClient

ui/aura/test/aura_test_helper.h

Index: ui/aura/test/aura_test_helper.h
diff --git a/ui/aura/test/aura_test_helper.h b/ui/aura/test/aura_test_helper.h
index 25f612bb5e2d8ffab3db08b9d37a44533928951b..f341c33b0abb5ef66fb5e4f4e253b8a2200c14e6 100644
--- a/ui/aura/test/aura_test_helper.h
+++ b/ui/aura/test/aura_test_helper.h
@@ -60,6 +60,11 @@ class AuraTestHelper {
   // before SetUp().
   void EnableMusWithWindowTreeClient(WindowTreeClient* window_tree_client);
 
+  // Explicitly destroy the WindowTreeClient. Normally the WindowTreeClient is
+  // shutdown at the right time and there is no need to call this. This is
+  // provided for testing shutdown ordering.
+  void DeleteWindowTreeClient();
+
   // Creates and initializes (shows and sizes) the RootWindow for use in tests.
   void SetUp(ui::ContextFactory* context_factory,
              ui::ContextFactoryPrivate* context_factory_private);