Remove the singleton instance get/delete methods from RootWindow (yay)

ash/shell.h

Index: ash/shell.h
===================================================================
--- ash/shell.h	(revision 124527)
+++ ash/shell.h	(working copy)
@@ -252,6 +252,8 @@
   // when the screen is initially created.
   static bool initially_hide_cursor_;
 
+  scoped_ptr<aura::RootWindow> root_window_;
+
   internal::RootWindowEventFilter* root_filter_;  // not owned
 
   std::vector<WindowAndBoundsPair> to_restore_;