Remove canTweakAlphaForCoverage from XP's since batch reads flag.

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
 
@@ skipping 10 matching lines @@
  */
 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;
     }
 
-    bool canTweakAlphaForCoverage() const override { return false; }
-
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;
 
 private:
     GrCoverageSetOpXPFactory(SkRegion::Op regionOp, bool invertCoverage);
 
     GrXferProcessor* onCreateXferProcessor(const GrDrawTargetCaps& caps,
                                            const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            const GrDeviceCoordTexture* dstCopy) const override;
@@ skipping 11 matching lines @@
 
     GR_DECLARE_XP_FACTORY_TEST;
 
     SkRegion::Op fRegionOp;
     bool         fInvertCoverage;
 
     typedef GrXPFactory INHERITED;
 };
 #endif