Use SkXfermode as public facing enum for GrPorterDuffXP

src/gpu/effects/GrCoverageSetOpXP.cpp

Index: src/gpu/effects/GrCoverageSetOpXP.cpp
diff --git a/src/gpu/effects/GrCoverageSetOpXP.cpp b/src/gpu/effects/GrCoverageSetOpXP.cpp
index f0ec1f9b00b4d7c74ce4431cb6dd551ab54c811e..2b262116f0ee08134472e34248fcb391bad5c929 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.cpp
+++ b/src/gpu/effects/GrCoverageSetOpXP.cpp
@@ -15,11 +15,6 @@
 #include "gl/builders/GrGLFragmentShaderBuilder.h"
 #include "gl/builders/GrGLProgramBuilder.h"
 
-/**
- * This xfer processor directly blends the the src coverage with the dst using a set operator. It is
- * useful for rendering coverage masks using CSG. It can optionally invert the src coverage before
- * applying the set operator.
- * */
 class CoverageSetOpXP : public GrXferProcessor {
 public:
     static GrXferProcessor* Create(SkRegion::Op regionOp, bool invertCoverage) {