Make XPFragmentBuilder only Builder with access to DstCopy.

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"
@@ skipping 48 matching lines @@
 
     ~GLDisableColorXP() override {}
 
     static void GenKey(const GrProcessor&, const GrGLSLCaps&, GrProcessorKeyBuilder*) {}
 
 private:
     void onEmitCode(const EmitArgs& args) override {
         // This emit code should be empty. However, on the nexus 6 there is a driver bug where if
         // you do not give gl_FragColor a value, the gl context is lost and we end up drawing
         // nothing. So this fix just sets the gl_FragColor arbitrarily to 0.
-        GrGLFPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+        GrGLXPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
         fsBuilder->codeAppendf("%s = vec4(0);", args.fOutputPrimary);
     }
 
     void onSetData(const GrGLProgramDataManager&, const GrXferProcessor&) override {}
 
     typedef GrGLXferProcessor INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
@@ skipping 29 matching lines @@
 
 GR_DEFINE_XP_FACTORY_TEST(GrDisableColorXPFactory);
 
 GrXPFactory* GrDisableColorXPFactory::TestCreate(SkRandom* random,
                                                   GrContext*,
                                                   const GrDrawTargetCaps&,
                                                   GrTexture*[]) {
     return GrDisableColorXPFactory::Create();
 }