Add gpu backend for SkPixelXorXfermode

include/effects/SkPixelXorXfermode.h

 /*
  * Copyright 2007 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkPixelXorXfermode_DEFINED
 #define SkPixelXorXfermode_DEFINED
 
 #include "SkXfermode.h"
 
 /** SkPixelXorXfermode implements a simple pixel xor (op ^ src ^ dst).
     This transformation does not follow premultiplied conventions, therefore
     this proc *always* returns an opaque color (alpha == 255). Thus it is
     not really usefull for operating on blended colors.
 */
 class SK_API SkPixelXorXfermode : public SkXfermode {
 public:
-    static SkXfermode* Create(SkColor opColor) { return new SkPixelXorXfermode(opColor); }
+    static SkXfermode* Create(SkColor opColor) {
+        return new SkPixelXorXfermode(opColor);
+    }
+
+#if SK_SUPPORT_GPU
+    bool asFragmentProcessor(const GrFragmentProcessor** output,
+                             const GrFragmentProcessor* dst) const override;
+
+    bool asXPFactory(GrXPFactory**) const override;
+#endif
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkPixelXorXfermode)
 
 protected:
     void flatten(SkWriteBuffer&) const override;
     SkPMColor xferColor(SkPMColor src, SkPMColor dst) const override;
 
 private:
     explicit SkPixelXorXfermode(SkColor opColor) : fOpColor(opColor) {}
 
     SkColor fOpColor;
 
     SkValue asValue() const override;
 
     typedef SkXfermode INHERITED;
 };
 
 #endif