Don't create a GXPFactory when blend is SrcOver

BUG=skia:

Review URL: https://codereview.chromium.org/1471053002
diff --git a/src/effects/SkLightingImageFilter.cpp b/src/effects/SkLightingImageFilter.cpp
index 4bb755e..9b6a20f 100644
--- a/src/effects/SkLightingImageFilter.cpp
+++ b/src/effects/SkLightingImageFilter.cpp
@@ -354,6 +354,7 @@
     GrPaint paint;
     GrFragmentProcessor* fp = this->getFragmentProcessor(src, matrix, bounds, boundaryMode);
     paint.addColorFragmentProcessor(fp)->unref();
+    paint.setPorterDuffXPFactory(SkXfermode::kSrc_Mode);
     drawContext->fillRectToRect(clip, paint, SkMatrix::I(), dstRect, srcRect);
 }