Don't create a GXPFactory when blend is SrcOver

src/effects/SkBlurMaskFilter.cpp

Index: src/effects/SkBlurMaskFilter.cpp
diff --git a/src/effects/SkBlurMaskFilter.cpp b/src/effects/SkBlurMaskFilter.cpp
index 0dedf0208c1be706b036f34775249550fea245db..8dd82c5ac369851076dc21ef424e28d699dc655e 100644
--- a/src/effects/SkBlurMaskFilter.cpp
+++ b/src/effects/SkBlurMaskFilter.cpp
@@ -1262,6 +1262,8 @@ bool SkBlurMaskFilterImpl::filterMaskGPU(GrTexture* src,
             // outer:  dst = dst * (1 - src)
             //             = 0 * src + (1 - src) * dst
             paint.setCoverageSetOpXPFactory(SkRegion::kDifference_Op);
+        } else {
+            paint.setCoverageSetOpXPFactory(SkRegion::kReplace_Op);
         }
 
         SkAutoTUnref<GrDrawContext> drawContext(context->drawContext((*result)->asRenderTarget()));