Remove canTweakAlphaForCoverage from XP's since batch reads flag.

src/gpu/effects/GrDisableColorXP.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 GrDisableColorXP_DEFINED
 #define GrDisableColorXP_DEFINED
 
 #include "GrTypes.h"
 #include "GrXferProcessor.h"
 
 class GrProcOptInfo;
 
 class GrDisableColorXPFactory : public GrXPFactory {
 public:
     static GrXPFactory* Create() {
         return SkNEW(GrDisableColorXPFactory);
     }
 
     bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const override {
         return true;
     }
 
-    bool canTweakAlphaForCoverage() const override { return true; }
-
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput* output) const override {
         output->fBlendedColorFlags = 0;
         output->fWillBlendWithDst = 0;
     }
 
 private:
     GrDisableColorXPFactory();
 
     GrXferProcessor* onCreateXferProcessor(const GrDrawTargetCaps& caps,
@@ skipping 11 matching lines @@
         return true;
     }
 
     GR_DECLARE_XP_FACTORY_TEST;
 
     typedef GrXPFactory INHERITED;
 };
 
 #endif