Do some minor pre cleanup work before converting all xfermodes to XPs.

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index dc0dd88402f81afbd52fe99c6b368bb0636ce443..a673dfdd7f449f4e8fa0d620979aafd9a6237906 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -704,7 +704,7 @@ bool SkXfermode::AsFragmentProcessorOrXPFactory(SkXfermode* xfermode,
     } else if (xfermode->asXPFactory(xpf)) {
         return true;
     } else {
-        return xfermode->asFragmentProcessor(fp);
+        return xfermode->asFragmentProcessor(fp, NULL);
     }
 }
 #else