Changes how window manager obtains WindowTree from mus

ash/mus/root_windows_observer.h

Index: ash/mus/root_windows_observer.h
diff --git a/ash/mus/root_windows_observer.h b/ash/mus/root_windows_observer.h
index 9df111159a945cce0b7168a63f72982469352be6..9ef27e5c833922289f88f70e453f2e48f3b49f8e 100644
--- a/ash/mus/root_windows_observer.h
+++ b/ash/mus/root_windows_observer.h
@@ -13,8 +13,11 @@ class RootWindowController;
 class RootWindowsObserver {
  public:
   // Called once a new display has been created and the root Window obtained.
-  virtual void OnRootWindowControllerAdded(
-      RootWindowController* controller) = 0;
+  virtual void OnRootWindowControllerAdded(RootWindowController* controller) {}
+
+  // Called before a RootWindowController is destroyed.
+  virtual void OnWillDestroyRootWindowController(
+      RootWindowController* controller) {}
 
  protected:
   ~RootWindowsObserver() {}