Check WindowTreeClient for empty inflight queue upon teardown

components/mus/public/cpp/tests/window_tree_client_private.h

Index: components/mus/public/cpp/tests/window_tree_client_private.h
diff --git a/components/mus/public/cpp/tests/window_tree_client_private.h b/components/mus/public/cpp/tests/window_tree_client_private.h
index 7cec901b36071e80286c757c64ebedd12f2393cb..197fede774412085e6e14b41ba643577b111461b 100644
--- a/components/mus/public/cpp/tests/window_tree_client_private.h
+++ b/components/mus/public/cpp/tests/window_tree_client_private.h
@@ -38,6 +38,16 @@ class WindowTreeClientPrivate {
   // Pretends that |event| has been received from the window server.
   void CallOnWindowInputEvent(Window* window, std::unique_ptr<ui::Event> event);
 
+  // For tests which do not connect to the Mus Window Server, the stubbed
+  // connection does not respond to messages. This clears the in flight queue of
+  // requests so that tear down can occur.
+  void ClearInFlightQueue();
+
+  // For tests which do connect to the Mus Window Server. This runs until the
+  // |tree_client_impl_| has received acknowledgements for each message sent to
+  // the server.
+  void WaitForInFlightQueue();
+
  private:
    WindowTreeClient* tree_client_impl_;