Make XPFragmentBuilder only Builder with access to DstCopy.
Plus a bunch of renaming.

BUG=skia:

Review URL: https://codereview.chromium.org/1110033004
diff --git a/src/gpu/GrAAConvexPathRenderer.cpp b/src/gpu/GrAAConvexPathRenderer.cpp
index 01241c4..f031f7f 100644
--- a/src/gpu/GrAAConvexPathRenderer.cpp
+++ b/src/gpu/GrAAConvexPathRenderer.cpp
@@ -565,7 +565,7 @@
             this->emitTransforms(args.fPB, gpArgs->fPositionVar, qe.inPosition()->fName,
                                  qe.localMatrix(), args.fTransformsIn, args.fTransformsOut);
 
-            GrGLGPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+            GrGLFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
 
             SkAssertResult(fsBuilder->enableFeature(
                     GrGLFragmentShaderBuilder::kStandardDerivatives_GLSLFeature));