Use static XPF for porter duff xp factories.

include/gpu/GrXferProcessor.h

Index: include/gpu/GrXferProcessor.h
diff --git a/include/gpu/GrXferProcessor.h b/include/gpu/GrXferProcessor.h
index 139ac96a995fca411ac1ba88355d661edd40087d..ee3764deda89a31d3970ebf00b4526e169bdbc53 100644
--- a/include/gpu/GrXferProcessor.h
+++ b/include/gpu/GrXferProcessor.h
@@ -43,7 +43,7 @@ private:
  * creates will have. For example, can it create an XP that supports RGB coverage or will the XP
  * blend with the destination color.
  */
-class GrXPFactory : public GrProgramElement {
+class GrXPFactory : public SkRefCnt {
 public:
     virtual const GrXferProcessor* createXferProcessor() const = 0;