Move GetCursor() method from WindowDelegate to its own delegate interface CursorDelegate.

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index 3b1fbce32464198103344831a8c90055172747f7..50585c8a011329d15bc5acfebc97014c2d21f2ec 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -221,9 +221,6 @@ class EmptyWindowDelegate : public aura::WindowDelegate {
   virtual void OnBoundsChanged(const gfx::Rect& old_bounds,
                                const gfx::Rect& new_bounds) OVERRIDE {
   }
-  virtual gfx::NativeCursor GetCursor(const gfx::Point& point) OVERRIDE {
-    return gfx::kNullCursor;
-  }
   virtual int GetNonClientComponent(
       const gfx::Point& point) const OVERRIDE {
     return HTNOWHERE;