Don't create a GXPFactory when blend is SrcOver

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 303 matching lines @@
     if (this->hasHWBlendEquation()) {
         blendInfo->fEquation = this->hwBlendEquation();
     }
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 class CustomXPFactory : public GrXPFactory {
 public:
     CustomXPFactory(SkXfermode::Mode mode);
 
-    bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const override {
-        return true;
-    }
-
     void getInvariantBlendedColor(const GrProcOptInfo& colorPOI,
                                   GrXPFactory::InvariantBlendedColor*) const override;
 
 private:
     GrXferProcessor* onCreateXferProcessor(const GrCaps& caps,
                                            const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            bool hasMixedSamples,
                                            const DstTexture*) const override;
 
@@ skipping 57 matching lines @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
 GrXPFactory* GrCustomXfermode::CreateXPFactory(SkXfermode::Mode mode) {
     if (!GrCustomXfermode::IsSupportedMode(mode)) {
         return nullptr;
     } else {
         return new CustomXPFactory(mode);
     }
 }