mus: Fix ash::GetRootWindowController for child windows/widgets

ash/root_window_controller.h

Index: ash/root_window_controller.h
diff --git a/ash/root_window_controller.h b/ash/root_window_controller.h
index 96b6ea2299819fc0790776e21b6cfa4469d016c3..a45ce1804299a3d962880a3bf760e75e6337974c 100644
--- a/ash/root_window_controller.h
+++ b/ash/root_window_controller.h
@@ -306,8 +306,9 @@ class ASH_EXPORT RootWindowController : public ShellObserver {
   DISALLOW_COPY_AND_ASSIGN(RootWindowController);
 };
 
-
-// Gets the RootWindowController for |root_window|.
+// On classic ash, returns the RootWindowController for the given |root_window|.
+// On mus ash, returns the RootWindowController for the primary display.
+// See RootWindowController class comment above.
 ASH_EXPORT RootWindowController* GetRootWindowController(
     const aura::Window* root_window);