Make XPFragmentBuilder only Builder with access to DstCopy.

src/gpu/effects/GrConvolutionEffect.cpp

Index: src/gpu/effects/GrConvolutionEffect.cpp
diff --git a/src/gpu/effects/GrConvolutionEffect.cpp b/src/gpu/effects/GrConvolutionEffect.cpp
index 336778e0371e7c80c75ed00b145a2c89bfa199d7..3d96a00e9d9aeea0a2b38a5fe618d0f60bc62415 100644
--- a/src/gpu/effects/GrConvolutionEffect.cpp
+++ b/src/gpu/effects/GrConvolutionEffect.cpp
@@ -69,7 +69,7 @@ void GrGLConvolutionEffect::emitCode(GrGLFPBuilder* builder,
                                           kFloat_GrSLType, kDefault_GrSLPrecision,
                                           "Kernel", this->width());
 
-    GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+    GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
     SkString coords2D = fsBuilder->ensureFSCoords2D(coords, 0);
 
     fsBuilder->codeAppendf("\t\t%s = vec4(0, 0, 0, 0);\n", outputColor);