Makes it possible for clients to directly create WindowTreeHostMus

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 a76aa170ad55e28da77c0f53610d7652ad72309c..ed8c86e6d407d922cfa104071e11f08d1dacd6e8 100644
--- a/ui/aura/test/aura_test_base.h
+++ b/ui/aura/test/aura_test_base.h
@@ -87,7 +87,7 @@ class AuraTestBase : public testing::Test,
   void SetPropertyConverter(std::unique_ptr<PropertyConverter> helper);
 
   // WindowTreeClientDelegate:
-  void OnEmbed(Window* root) override;
+  void OnEmbed(std::unique_ptr<WindowTreeHostMus> window_tree_host) override;
   void OnUnembed(Window* root) override;
   void OnEmbedRootDestroyed(Window* root) override;
   void OnLostConnection(WindowTreeClient* client) override;
@@ -105,7 +105,8 @@ class AuraTestBase : public testing::Test,
       std::map<std::string, std::vector<uint8_t>>* properties) override;
   void OnWmClientJankinessChanged(const std::set<Window*>& client_windows,
                                   bool janky) override;
-  void OnWmNewDisplay(Window* window, const display::Display& display) override;
+  void OnWmNewDisplay(std::unique_ptr<WindowTreeHostMus> window_tree_host,
+                      const display::Display& display) override;
   void OnWmDisplayRemoved(Window* window) override;
   void OnWmDisplayModified(const display::Display& display) override;
   ui::mojom::EventResult OnAccelerator(uint32_t id,