Provide a GPU implementation of SkArithmeticMode

include/effects/SkArithmeticMode.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkArithmeticMode_DEFINED
 #define SkArithmeticMode_DEFINED
 
 #include "SkXfermode.h"
 
 class SK_API SkArithmeticMode : public SkXfermode {
 public:
     /**
      *  result = clamp[k1 * src * dst + k2 * src + k3 * dst + k4]
      *
      *  src and dst are treated as being [0.0 .. 1.0]. The polynomial is
      *  evaluated on their unpremultiplied components.
      *
      *  k1=k2=k3=0, k4=1.0 results in returning opaque white
      *  k1=k3=k4=0, k2=1.0 results in returning the src
      *  k1=k2=k4=0, k3=1.0 results in returning the dst
      */
     static SkXfermode* Create(SkScalar k1, SkScalar k2,
                               SkScalar k3, SkScalar k4);
 
+    SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP();
+
 private:
     typedef SkXfermode INHERITED;
 };
 
 #endif