Don't create a GXPFactory when blend is SrcOver

tests/TessellatingPathRendererTests.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkPath.h"
 
 #if SK_SUPPORT_GPU
@@ skipping 218 matching lines @@
     path.lineTo(    0.0f,  -1.0f);
     path.lineTo(10000.0f,   0.000001f);
     path.lineTo(    0.0f, -30.0f);
     return path;
 }
 
 static void test_path(GrDrawTarget* dt, GrRenderTarget* rt, GrResourceProvider* rp,
                       const SkPath& path) {
     GrTessellatingPathRenderer tess;
     GrPipelineBuilder pipelineBuilder;
+    pipelineBuilder.setXPFactory(
+        GrPorterDuffXPFactory::Create(SkXfermode::kSrc_Mode))->unref();
     pipelineBuilder.setRenderTarget(rt);
     GrStrokeInfo stroke(SkStrokeRec::kFill_InitStyle);
     GrPathRenderer::DrawPathArgs args;
     args.fTarget = dt;
     args.fPipelineBuilder = &pipelineBuilder;
     args.fResourceProvider = rp;
     args.fColor = GrColor_WHITE;
     args.fViewMatrix = &SkMatrix::I();
     args.fPath = &path;
     args.fStroke = &stroke;
@@ skipping 30 matching lines @@
     test_path(dt, rt, rp, create_path_8());
     test_path(dt, rt, rp, create_path_9());
     test_path(dt, rt, rp, create_path_10());
     test_path(dt, rt, rp, create_path_11());
     test_path(dt, rt, rp, create_path_12());
     test_path(dt, rt, rp, create_path_13());
     test_path(dt, rt, rp, create_path_14());
     test_path(dt, rt, rp, create_path_15());
 }
 #endif