Don't create a GXPFactory when blend is SrcOver

src/gpu/SkGr.cpp

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "SkGr.h"
 
@@ skipping 471 matching lines @@
             if (cfFP) {
                 grPaint->addColorFragmentProcessor(cfFP);
             } else {
                 return false;
             }
         }
     }
 
     SkXfermode* mode = skPaint.getXfermode();
     GrXPFactory* xpFactory = nullptr;
-    if (!SkXfermode::AsXPFactory(mode, &xpFactory)) {
-        // Fall back to src-over
-        // return false here?
-        xpFactory = GrPorterDuffXPFactory::Create(SkXfermode::kSrcOver_Mode);
-    }
-    SkASSERT(xpFactory);
-    grPaint->setXPFactory(xpFactory)->unref();
+    SkXfermode::AsXPFactory(mode, &xpFactory);
+    SkSafeUnref(grPaint->setXPFactory(xpFactory));
 
 #ifndef SK_IGNORE_GPU_DITHER
     if (skPaint.isDither() && grPaint->numColorFragmentProcessors() > 0) {
         grPaint->addColorFragmentProcessor(GrDitherEffect::Create())->unref();
     }
 #endif
     return true;
 }
 
 bool SkPaintToGrPaint(GrContext* context, const SkPaint& skPaint, const SkMatrix& viewM,
@@ skipping 123 matching lines @@
             SkErrorInternals::SetError( kInvalidPaint_SkError,
                                         "Sorry, I don't understand the filtering "
                                         "mode you asked for.  Falling back to "
                                         "MIPMaps.");
             textureFilterMode = GrTextureParams::kMipMap_FilterMode;
             break;
 
     }
     return textureFilterMode;
 }