Make XPFragmentBuilder only Builder with access to DstCopy.

src/effects/gradients/SkTwoPointRadialGradient.cpp

Index: src/effects/gradients/SkTwoPointRadialGradient.cpp
diff --git a/src/effects/gradients/SkTwoPointRadialGradient.cpp b/src/effects/gradients/SkTwoPointRadialGradient.cpp
index 0146a772327c9b33e56ac09549d6a3d2e41e51a5..44b73d6a3e256a78d77c4be7a26c0c41f24e8429 100644
--- a/src/effects/gradients/SkTwoPointRadialGradient.cpp
+++ b/src/effects/gradients/SkTwoPointRadialGradient.cpp
@@ -589,7 +589,7 @@ void GrGLRadial2Gradient::emitCode(GrGLFPBuilder* builder,
     builder->getUniformVariable(fParamUni).appendArrayAccess(4, &p4);
     builder->getUniformVariable(fParamUni).appendArrayAccess(5, &p5);
 
-    GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+    GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
     // We interpolate the linear component in coords[1].
     SkASSERT(coords[0].getType() == coords[1].getType());
     const char* coords2D;