Converts WindowTreeClientTest to be in terms of aura

ash/mus/window_manager_unittest.cc

Index: ash/mus/window_manager_unittest.cc
diff --git a/ash/mus/window_manager_unittest.cc b/ash/mus/window_manager_unittest.cc
index 51dac6bafb5d57efdee6430f4254bf71c4c282b7..095b1946f29f702a405d1a2eb6c149e7d41c91ea 100644
--- a/ash/mus/window_manager_unittest.cc
+++ b/ash/mus/window_manager_unittest.cc
@@ -43,7 +43,8 @@ class WindowTreeClientDelegate : public aura::WindowTreeClientDelegate {
     window_tree_host_ = std::move(window_tree_host);
     run_loop_.Quit();
   }
-  void OnEmbedRootDestroyed(aura::Window* root) override {}
+  void OnEmbedRootDestroyed(
+      aura::WindowTreeHostMus* window_tree_host) override {}
   void OnLostConnection(aura::WindowTreeClient* client) override {}
   void OnPointerEventObserved(const ui::PointerEvent& event,
                               aura::Window* target) override {}