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..df65e451feacfdfac1802f197a16f6cc48d29cc8 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -489,10 +489,15 @@ static inline bool skpaint_to_grpaint_impl(GrContext* context,
         }
     }
 
-    SkXfermode* mode = skPaint.getXfermode();
-    GrXPFactory* xpFactory = nullptr;
-    SkXfermode::AsXPFactory(mode, &xpFactory);
-    SkSafeUnref(grPaint->setXPFactory(xpFactory));
+    // When the xfermode is null on the SkPaint (meaning kSrcOver) we need the XPFactory field on
+    // the GrPaint to also be null (also kSrcOver).
+    SkASSERT(!grPaint->getXPFactory());
+    SkXfermode* xfermode = skPaint.getXfermode();
+    if (xfermode) {
+        // SafeUnref in case a new xfermode is added that returns null. 
+        // In such cases we will fall back to kSrcOver_Mode.
+        SkSafeUnref(grPaint->setXPFactory(xfermode->asXPFactory()));
+    }
 
 #ifndef SK_IGNORE_GPU_DITHER
     if (skPaint.isDither() && grPaint->numColorFragmentProcessors() > 0) {