Make GetRootWindow() return a Window instead of a RootWindow.

ash/wm/overview/scoped_transform_overview_window.h

Index: ash/wm/overview/scoped_transform_overview_window.h
diff --git a/ash/wm/overview/scoped_transform_overview_window.h b/ash/wm/overview/scoped_transform_overview_window.h
index 73ae309de1d304000c5bbebb71417b9de5bb1270..edd0ccb00dacd7a35c56bb8d784ed8bcb8b94013 100644
--- a/ash/wm/overview/scoped_transform_overview_window.h
+++ b/ash/wm/overview/scoped_transform_overview_window.h
@@ -10,7 +10,6 @@
 #include "ui/gfx/transform.h"
 
 namespace aura {
-class RootWindow;
 class Window;
 }
 
@@ -71,7 +70,7 @@ class ScopedTransformOverviewWindow {
   // Sets |transform| on the window and a copy of the window if the target
   // |root_window| is not the window's root window. If |animate| the transform
   // is animated in, otherwise it is immediately applied.
-  void SetTransform(aura::RootWindow* root_window,
+  void SetTransform(aura::Window* root_window,
                     const gfx::Transform& transform,
                     bool animate);