Make XPFragmentBuilder only Builder with access to DstCopy.

src/gpu/gl/GrGLXferProcessor.cpp

Index: src/gpu/gl/GrGLXferProcessor.cpp
diff --git a/src/gpu/gl/GrGLXferProcessor.cpp b/src/gpu/gl/GrGLXferProcessor.cpp
index b7bf3019b5921c3c5aca4d0aa2234f1a96f7f94f..c517381249ae77735346c5a050d026ab38467e20 100644
--- a/src/gpu/gl/GrGLXferProcessor.cpp
+++ b/src/gpu/gl/GrGLXferProcessor.cpp
@@ -15,7 +15,7 @@ void GrGLXferProcessor::emitCode(const EmitArgs& args) {
     if (args.fXP.getDstCopyTexture()) {
         bool topDown = kTopLeft_GrSurfaceOrigin == args.fXP.getDstCopyTexture()->origin();
 
-        GrGLFPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+        GrGLXPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
 
         // We don't think any shaders actually output negative coverage, but just as a safety check
         // for floating point precision errors we compare with <= here