Sky: Add a DrawLooper interface to the painting API to be used for shadows.

sky/engine/core/painting/Paint.idl

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   Constructor(),
 ] interface Paint {
   attribute boolean isAntiAlias;
   attribute unsigned long color;
 
   void setARGB(unsigned long a, unsigned long r, unsigned long g, unsigned long b);
+  void setDrawLooper(DrawLooper looper);
 };