Don't create a GXPFactory when blend is SrcOver

BUG=skia:

Review URL: https://codereview.chromium.org/1471053002
diff --git a/src/gpu/GrDrawTarget.cpp b/src/gpu/GrDrawTarget.cpp
index 062a42f..01d4f63 100644
--- a/src/gpu/GrDrawTarget.cpp
+++ b/src/gpu/GrDrawTarget.cpp
@@ -431,6 +431,8 @@
         }
 
         GrPipelineBuilder pipelineBuilder;
+        pipelineBuilder.setXPFactory(
+            GrPorterDuffXPFactory::Create(SkXfermode::kSrc_Mode))->unref();
         pipelineBuilder.setRenderTarget(renderTarget);
 
         this->drawNonAARect(pipelineBuilder, color, SkMatrix::I(), *rect);