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

include/core/SkXfermode.h

Index: include/core/SkXfermode.h
diff --git a/include/core/SkXfermode.h b/include/core/SkXfermode.h
index 05fd81152a72e1c6c2898991fd918641260c3566..ea0c6c6f38a77b19eadc766a4c29bd159433c2d3 100644
--- a/include/core/SkXfermode.h
+++ b/include/core/SkXfermode.h
@@ -199,10 +199,7 @@ public:
         fragment shader. If NULL, the effect should request access to destination color
         (setWillReadDstColor()), and use that in the fragment shader (builder->dstColor()).
      */
-    // TODO: Once all custom xp's have been created the background parameter will be required here.
-    //       We will always use the XPFactory if there is no background texture and the fragment if
-    //       there is one.
-    virtual bool asFragmentProcessor(GrFragmentProcessor**, GrTexture* background = NULL) const;
+    virtual bool asFragmentProcessor(GrFragmentProcessor**, GrTexture* background) const;
 
     /** A subclass may implement this factory function to work with the GPU backend. It is legal
         to call this with xpf NULL to simply test the return value. If xpf is non-NULL then the