Use static XPF for porter duff xp factories.

BUG=skia:

Review URL: https://codereview.chromium.org/776843004
diff --git a/src/gpu/GrDrawState.cpp b/src/gpu/GrDrawState.cpp
index 0fd0987..db70bfa 100644
--- a/src/gpu/GrDrawState.cpp
+++ b/src/gpu/GrDrawState.cpp
@@ -116,8 +116,7 @@
     fRenderTarget.reset(NULL);
 
     fGeometryProcessor.reset(NULL);
-    fXPFactory.reset(GrPorterDuffXPFactory::Create(kOne_GrBlendCoeff,
-                                                   kZero_GrBlendCoeff));
+    fXPFactory.reset(GrPorterDuffXPFactory::Create(SkXfermode::kSrc_Mode));
     fColorStages.reset();
     fCoverageStages.reset();