Converts WindowTreeClientTest to be in terms of aura

mash/simple_wm/simple_wm.cc

Index: mash/simple_wm/simple_wm.cc
diff --git a/mash/simple_wm/simple_wm.cc b/mash/simple_wm/simple_wm.cc
index 49917e298038468c406f6f578fbb3b05073c1366..8ff0659f1ba8f3a7b1eb74fc02899cac5aec822c 100644
--- a/mash/simple_wm/simple_wm.cc
+++ b/mash/simple_wm/simple_wm.cc
@@ -114,7 +114,7 @@ void SimpleWM::OnLostConnection(aura::WindowTreeClient* client) {
   window_tree_client_.reset();
 }
 
-void SimpleWM::OnEmbedRootDestroyed(aura::Window* root) {
+void SimpleWM::OnEmbedRootDestroyed(aura::WindowTreeHostMus* window_tree_host) {
   // WindowTreeClients configured as the window manager should never get
   // OnEmbedRootDestroyed().
   NOTREACHED();