Remove canTweakAlphaForCoverage from XP's since batch reads flag.

src/gpu/effects/GrCustomXfermodePriv.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrCustomXfermodePriv_DEFINED
 #define GrCustomXfermodePriv_DEFINED
 
@@ skipping 46 matching lines @@
 ///////////////////////////////////////////////////////////////////////////////
 
 class GrCustomXPFactory : public GrXPFactory {
 public:
     GrCustomXPFactory(SkXfermode::Mode mode); 
 
     bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const override {
         return true;
     }
 
-    bool canTweakAlphaForCoverage() const override {
-        return false;
-    }
-
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;
 
 private:
     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 {
         return true;
     }
 
     bool onIsEqual(const GrXPFactory& xpfBase) const override {
         const GrCustomXPFactory& xpf = xpfBase.cast<GrCustomXPFactory>();
         return fMode == xpf.fMode;
     }
 
     GR_DECLARE_XP_FACTORY_TEST;
 
     SkXfermode::Mode fMode;
 
     typedef GrXPFactory INHERITED;
 };
 #endif