Add more specialized fragment builders

Adds specialized fragment builders for primitive and fragment
processors. Removes fragment-specific functionality from the Xfer
fragment builder.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1709153002

Review URL: https://codereview.chromium.org/1709153002
diff --git a/src/gpu/effects/GrConfigConversionEffect.cpp b/src/gpu/effects/GrConfigConversionEffect.cpp
index ace736c..4e168cd 100644
--- a/src/gpu/effects/GrConfigConversionEffect.cpp
+++ b/src/gpu/effects/GrConfigConversionEffect.cpp
@@ -26,7 +26,7 @@
         SkString tmpDecl;
         tmpVar.appendDecl(args.fGLSLCaps, &tmpDecl);
 
-        GrGLSLFragmentBuilder* fragBuilder = args.fFragBuilder;
+        GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
 
         fragBuilder->codeAppendf("%s;", tmpDecl.c_str());