Make specialized SrcOver XPFactory

src/gpu/effects/GrCustomXfermode.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "effects/GrCustomXfermode.h"
 
 #include "GrCoordTransform.h"
@@ skipping 377 matching lines @@
 GR_DEFINE_XP_FACTORY_TEST(CustomXPFactory);
 const GrXPFactory* CustomXPFactory::TestCreate(GrProcessorTestData* d) {
     int mode = d->fRandom->nextRangeU(SkXfermode::kLastCoeffMode + 1,
                                       SkXfermode::kLastSeparableMode);
 
     return new CustomXPFactory(static_cast<SkXfermode::Mode>(mode));
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
-GrXPFactory* GrCustomXfermode::CreateXPFactory(SkXfermode::Mode mode) {
+const GrXPFactory* GrCustomXfermode::CreateXPFactory(SkXfermode::Mode mode) {
     if (!GrCustomXfermode::IsSupportedMode(mode)) {
         return nullptr;
     } else {
         return new CustomXPFactory(mode);
     }
 }