Don't create a GXPFactory when blend is SrcOver

include/gpu/effects/GrCoverageSetOpXP.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrCoverageSetOpXP_DEFINED
 #define GrCoverageSetOpXP_DEFINED
 
 #include "GrTypes.h"
 #include "GrXferProcessor.h"
 #include "SkRegion.h"
 
 class GrProcOptInfo;
 
 /**
  * This xfer processor directly blends the the src coverage with the dst using a set operator. It is
  * useful for rendering coverage masks using CSG. It can optionally invert the src coverage before
  * applying the set operator.
  */
 class GrCoverageSetOpXPFactory : public GrXPFactory {
 public:
     static GrXPFactory* Create(SkRegion::Op regionOp, bool invertCoverage = false);
 
-    bool supportsRGBCoverage(GrColor /*knownColor*/,
-                             uint32_t /*knownColorFlags*/) const override {
-        return true;
-    }
-
     void getInvariantBlendedColor(const GrProcOptInfo& colorPOI,
                                   GrXPFactory::InvariantBlendedColor*) const override;
 
 private:
     GrCoverageSetOpXPFactory(SkRegion::Op regionOp, bool invertCoverage);
 
     GrXferProcessor* onCreateXferProcessor(const GrCaps& caps,
                                            const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            bool hasMixedSamples,
@@ skipping 13 matching lines @@
 
     GR_DECLARE_XP_FACTORY_TEST;
 
     SkRegion::Op fRegionOp;
     bool         fInvertCoverage;
 
     typedef GrXPFactory INHERITED;
 };
 #endif