Add gpu implementation of OverdrawXfermode

src/effects/SkArithmeticMode.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkArithmeticMode.h"
 #include "SkColorPriv.h"
 #include "SkReadBuffer.h"
@@ skipping 16 matching lines @@
             SkScalarNearlyZero(k3) && SkScalarNearlyZero(k4)) {
             return SkXfermode::Create(SkXfermode::kSrc_Mode);
         } else if (SkScalarNearlyZero(k1) && SkScalarNearlyZero(k2) && 
                    SkScalarNearlyEqual(k3, SK_Scalar1) && SkScalarNearlyZero(k4)) {
             return SkXfermode::Create(SkXfermode::kDst_Mode);
         }
 
         return new SkArithmeticMode_scalar(k1, k2, k3, k4, enforcePMColor);
     }
 
-    virtual void xfer32(SkPMColor dst[], const SkPMColor src[], int count,
-                        const SkAlpha aa[]) const override;
+    void xfer32(SkPMColor dst[], const SkPMColor src[], int count,
+                const SkAlpha aa[]) const override;
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkArithmeticMode_scalar)
 
 #if SK_SUPPORT_GPU
     bool asFragmentProcessor(const GrFragmentProcessor**,
                              const GrFragmentProcessor* dst) const override;
 
     bool asXPFactory(GrXPFactory**) const override;
 #endif
@@ skipping 230 matching lines @@
                                              fEnforcePMColor);
     }
     return true;
 }
 
 #endif
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkArithmeticMode)
     SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkArithmeticMode_scalar)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END