Fixes to WindowTreeHostMus

ui/aura/test/mus/test_window_tree.h

Index: ui/aura/test/mus/test_window_tree.h
diff --git a/ui/aura/test/mus/test_window_tree.h b/ui/aura/test/mus/test_window_tree.h
index aee9c5596dba47cbfc5a35aa7315321dada498e1..ce6f7ec622501c36444e0e34da88004bb6ade589 100644
--- a/ui/aura/test/mus/test_window_tree.h
+++ b/ui/aura/test/mus/test_window_tree.h
@@ -67,6 +67,8 @@ class TestWindowTree : public ui::mojom::WindowTree {
   bool GetAndRemoveFirstChangeOfType(WindowTreeChangeType type,
                                      uint32_t* change_id);
 
+  size_t GetChangeCountForType(WindowTreeChangeType type);
+
  private:
   struct Change {
     WindowTreeChangeType type;