Makes GetRootWindow() public on Window so we can begin to convert uses of RootWindow::GetInstance().

ui/aura/root_window.h

Index: ui/aura/root_window.h
===================================================================
--- ui/aura/root_window.h	(revision 119894)
+++ ui/aura/root_window.h	(working copy)
@@ -180,14 +180,15 @@
   // Provided only for testing:
   void SetGestureRecognizerForTesting(GestureRecognizer* gr);
 
-  // Overridden from Window:
-  virtual void SetTransform(const ui::Transform& transform) OVERRIDE;
-
 #if !defined(NDEBUG)
   // Toggles the host's full screen state.
   void ToggleFullScreen();
 #endif
 
+  // Overridden from Window:
+  virtual RootWindow* GetRootWindow() OVERRIDE;
+  virtual void SetTransform(const ui::Transform& transform) OVERRIDE;
+
   // Overridden from ui::CompositorDelegate:
   virtual void ScheduleDraw() OVERRIDE;
 
@@ -215,7 +216,6 @@
   virtual bool CanFocus() const OVERRIDE;
   virtual bool CanReceiveEvents() const OVERRIDE;
   virtual internal::FocusManager* GetFocusManager() OVERRIDE;
-  virtual RootWindow* GetRootWindow() OVERRIDE;
 
   // Overridden from ui::LayerAnimationObserver:
   virtual void OnLayerAnimationEnded(