chromeos: makes it possible for windows to outlive WindowTreeClient

ui/aura/test/aura_test_base.h

Index: ui/aura/test/aura_test_base.h
diff --git a/ui/aura/test/aura_test_base.h b/ui/aura/test/aura_test_base.h
index 34e9bca46a864613195b04a83e12b47a01e98953..5e492566f87b030431061132de4b66873f23acb5 100644
--- a/ui/aura/test/aura_test_base.h
+++ b/ui/aura/test/aura_test_base.h
@@ -64,6 +64,11 @@ class AuraTestBase : public testing::Test,
   // Turns on mus with a test WindowTree. Must be called before SetUp().
   void EnableMusWithTestWindowTree();
 
+  // 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();
+
   // Used to configure the backend. This is exposed to make parameterized tests
   // easy to write. This *must* be called from SetUp().
   void ConfigureBackend(BackendType type);