Add support for MouseEnter/MouseExit event types to be sent to aura::Windows.

ui/aura/root_window.h

Index: ui/aura/root_window.h
===================================================================
--- ui/aura/root_window.h	(revision 102135)
+++ ui/aura/root_window.h	(working copy)
@@ -50,7 +50,12 @@
   virtual internal::RootWindow* GetRoot() OVERRIDE;
 
  private:
+  // Called whenever the mouse moves, tracks the current |mouse_moved_handler_|,
+  // sending exited and entered events as its value changes.
+  void HandleMouseMoved(const MouseEvent& event, Window* target);
+
   Window* mouse_pressed_handler_;
+  Window* mouse_moved_handler_;
   scoped_ptr<FocusManager> focus_manager_;
   Window* capture_window_;