Remove canTweakAlphaForCoverage from XP's since batch reads flag.

include/gpu/effects/GrPorterDuffXferProcessor.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 GrPorterDuffXferProcessor_DEFINED
 #define GrPorterDuffXferProcessor_DEFINED
 
 #include "GrTypes.h"
 #include "GrXferProcessor.h"
 #include "SkXfermode.h"
 
 class GrProcOptInfo;
 
 class GrPorterDuffXPFactory : public GrXPFactory {
 public:
     static GrXPFactory* Create(SkXfermode::Mode mode); 
 
     bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const override;
 
-    bool canTweakAlphaForCoverage() const override;
-
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;
 
 private:
     GrPorterDuffXPFactory(GrBlendCoeff src, GrBlendCoeff dst); 
 
     GrXferProcessor* onCreateXferProcessor(const GrDrawTargetCaps& caps,
                                            const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            const GrDeviceCoordTexture* dstCopy) const override;
 
     bool willReadDstColor(const GrDrawTargetCaps& caps,
                           const GrProcOptInfo& colorPOI,
                           const GrProcOptInfo& coveragePOI) const override;
 
     bool onIsEqual(const GrXPFactory& xpfBase) const override {
         const GrPorterDuffXPFactory& xpf = xpfBase.cast<GrPorterDuffXPFactory>();
         return (fSrcCoeff == xpf.fSrcCoeff && fDstCoeff == xpf.fDstCoeff);
     }
 
     GR_DECLARE_XP_FACTORY_TEST;
 
     GrBlendCoeff fSrcCoeff;
     GrBlendCoeff fDstCoeff;
 
     typedef GrXPFactory INHERITED;
 };
 
 #endif