Don't create a GXPFactory when blend is SrcOver

src/gpu/GrYUVProvider.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 "GrContext.h"
 #include "GrDrawContext.h"
 #include "GrYUVProvider.h"
@@ skipping 110 matching lines @@
     SkASSERT(renderTarget);
 
     GrPaint paint;
     SkAutoTUnref<GrFragmentProcessor> yuvToRgbProcessor(
                                         GrYUVtoRGBEffect::Create(yuvTextures[0],
                                                                  yuvTextures[1],
                                                                  yuvTextures[2],
                                                                  yuvInfo.fSize,
                                                                  yuvInfo.fColorSpace));
     paint.addColorFragmentProcessor(yuvToRgbProcessor);
+    paint.setPorterDuffXPFactory(SkXfermode::kSrc_Mode);
     const SkRect r = SkRect::MakeIWH(yuvInfo.fSize[0].fWidth, yuvInfo.fSize[0].fHeight);
 
     SkAutoTUnref<GrDrawContext> drawContext(ctx->drawContext(renderTarget));
     if (!drawContext) {
         return nullptr;
     }
 
     drawContext->drawRect(GrClip::WideOpen(), paint, SkMatrix::I(), r);
 
     return result.detach();
 }