Make XPFragmentBuilder only Builder with access to DstCopy.

src/gpu/effects/GrConstColorProcessor.cpp

Index: src/gpu/effects/GrConstColorProcessor.cpp
diff --git a/src/gpu/effects/GrConstColorProcessor.cpp b/src/gpu/effects/GrConstColorProcessor.cpp
index c48fc55abc4b3f6d599d6fb95e386f38497c9c83..ab703fdd907376ccd30b54f24521dd34cb6f182e 100644
--- a/src/gpu/effects/GrConstColorProcessor.cpp
+++ b/src/gpu/effects/GrConstColorProcessor.cpp
@@ -20,7 +20,7 @@ public:
                   const char* inputColor,
                   const TransformedCoordsArray& coords,
                   const TextureSamplerArray& samplers) override {
-        GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+        GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
         const char* colorUni;
         fColorUniform = builder->addUniform(GrGLProgramBuilder::kFragment_Visibility,
                                             kVec4f_GrSLType, kMedium_GrSLPrecision, "constantColor",