Use static XPF for porter duff xp factories.

BUG=skia:

Review URL: https://codereview.chromium.org/776843004
diff --git a/src/gpu/GrPaint.cpp b/src/gpu/GrPaint.cpp
index bd5b286..443e1c0 100644
--- a/src/gpu/GrPaint.cpp
+++ b/src/gpu/GrPaint.cpp
@@ -52,8 +52,7 @@
 void GrPaint::resetStages() {
     fColorStages.reset();
     fCoverageStages.reset();
-    fXPFactory.reset(GrPorterDuffXPFactory::Create(kOne_GrBlendCoeff,
-                                                   kZero_GrBlendCoeff));
+    fXPFactory.reset(GrPorterDuffXPFactory::Create(SkXfermode::kSrcOver_Mode));
 }
 
 bool GrPaint::getOpaqueAndKnownColor(GrColor* solidColor,