Apply coverage in XP base class when using dst reads

src/gpu/effects/GrDisableColorXP.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "effects/GrDisableColorXP.h"
 #include "GrProcessor.h"
 #include "gl/GrGLXferProcessor.h"
 #include "gl/builders/GrGLFragmentShaderBuilder.h"
 #include "gl/builders/GrGLProgramBuilder.h"
 
 /**
  * This xfer processor disables color writing. Thus color and coverage and ignored and no blending
  * occurs. This XP is usful for things like stenciling.
  */
 class DisableColorXP : public GrXferProcessor {
 public:
     static GrXferProcessor* Create() {
         return SkNEW(DisableColorXP);
     }
 
     ~DisableColorXP() override {};
 
     const char* name() const override { return "Disable Color"; }
 
     GrGLXferProcessor* createGLInstance() const override;
 
-    bool hasSecondaryOutput() const override { return false; }
-
 private:
     DisableColorXP();
 
     GrXferProcessor::OptFlags onGetOptimizations(const GrProcOptInfo& colorPOI,
                                                  const GrProcOptInfo& coveragePOI,
                                                  bool doesStencilWrite,
                                                  GrColor* color,
                                                  const GrCaps& caps) override {
         return GrXferProcessor::kIgnoreColor_OptFlag | GrXferProcessor::kIgnoreCoverage_OptFlag;
     }
@@ skipping 67 matching lines @@
 
 GR_DEFINE_XP_FACTORY_TEST(GrDisableColorXPFactory);
 
 GrXPFactory* GrDisableColorXPFactory::TestCreate(SkRandom* random,
                                                   GrContext*,
                                                   const GrCaps&,
                                                   GrTexture*[]) {
     return GrDisableColorXPFactory::Create();
 }