mash: remove more shell/shelf WmWindow usage.

ash/mus/top_level_window_factory.cc

Index: ash/mus/top_level_window_factory.cc
diff --git a/ash/mus/top_level_window_factory.cc b/ash/mus/top_level_window_factory.cc
index 7fa50bdbf485eed84c23fccffd5e70ab658564d0..8149f63acc6f362078dd0c26fb06f5abef317f7f 100644
--- a/ash/mus/top_level_window_factory.cc
+++ b/ash/mus/top_level_window_factory.cc
@@ -72,8 +72,7 @@ RootWindowController* GetRootWindowControllerForNewTopLevelWindow(
       }
     }
   }
-  return RootWindowController::ForWindow(
-      Shell::GetWmRootWindowForNewWindows()->aura_window());
+  return GetRootWindowController(Shell::GetRootWindowForNewWindows());
 }
 
 // Returns the bounds for the new window.