Make XPFragmentBuilder only Builder with access to DstCopy.

src/gpu/effects/GrDitherEffect.cpp

Index: src/gpu/effects/GrDitherEffect.cpp
diff --git a/src/gpu/effects/GrDitherEffect.cpp b/src/gpu/effects/GrDitherEffect.cpp
index b692de8a0628d91994ea394f2ab0a49504e928b9..85116aee28cde08afd465df045b96d7037032f93 100644
--- a/src/gpu/effects/GrDitherEffect.cpp
+++ b/src/gpu/effects/GrDitherEffect.cpp
@@ -87,7 +87,7 @@ void GLDitherEffect::emitCode(GrGLFPBuilder* builder,
                               const char* inputColor,
                               const TransformedCoordsArray&,
                               const TextureSamplerArray& samplers) {
-    GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+    GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
     // Generate a random number based on the fragment position. For this
     // random number generator, we use the "GLSL rand" function
     // that seems to be floating around on the internet. It works under