Alter SkXfermode's asFragmentProcessor & asXPFactory contracts

src/gpu/SkGr.cpp

Index: src/gpu/SkGr.cpp
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index e4dc467e7f8b9ce2ccd27d5957f505371a515063..c89229a8a4c6b41d698fa94b8f1c2e2f9d24871f 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -489,10 +489,10 @@ static inline bool skpaint_to_grpaint_impl(GrContext* context,
         }
     }
 
-    SkXfermode* mode = skPaint.getXfermode();
-    GrXPFactory* xpFactory = nullptr;
-    SkXfermode::AsXPFactory(mode, &xpFactory);
-    SkSafeUnref(grPaint->setXPFactory(xpFactory));
+    SkXfermode* xfermode = skPaint.getXfermode();
+    if (xfermode) {

[egdaniel, 2016/02/08 15:32:11]

This is the check for srcOver right? So that we keep it null on the GrPaint? If
that is the case I think we may want an assert the before this call the XPF on
GrPaint is null. Along with a comment explaining why this is here?

[robertphillips, 2016/02/08 15:54:53]

Done.

+        grPaint->setXPFactory(xfermode->asXPFactory())->unref();
+    }
 
 #ifndef SK_IGNORE_GPU_DITHER
     if (skPaint.isDither() && grPaint->numColorFragmentProcessors() > 0) {