Don't create a GXPFactory when blend is SrcOver

gm/convexpolyeffect.cpp

Index: gm/convexpolyeffect.cpp
diff --git a/gm/convexpolyeffect.cpp b/gm/convexpolyeffect.cpp
index 4f3843672691bccf46615473c9a0a6ccbb8fb497..f040b90ed28a56cbb110948a600a0beae9304d51 100644
--- a/gm/convexpolyeffect.cpp
+++ b/gm/convexpolyeffect.cpp
@@ -190,6 +190,8 @@ protected:
                 }
 
                 GrPipelineBuilder pipelineBuilder;
+                pipelineBuilder.setXPFactory(
+                    GrPorterDuffXPFactory::Create(SkXfermode::kSrc_Mode))->unref();
                 pipelineBuilder.addCoverageFragmentProcessor(fp);
                 pipelineBuilder.setRenderTarget(rt);
 
@@ -239,6 +241,8 @@ protected:
                 }
 
                 GrPipelineBuilder pipelineBuilder;
+                pipelineBuilder.setXPFactory(
+                    GrPorterDuffXPFactory::Create(SkXfermode::kSrc_Mode))->unref();
                 pipelineBuilder.addCoverageFragmentProcessor(fp);
                 pipelineBuilder.setRenderTarget(rt);