Don't ref/unref the static src-over xp

include/gpu/effects/GrPorterDuffXferProcessor.h

Index: include/gpu/effects/GrPorterDuffXferProcessor.h
diff --git a/include/gpu/effects/GrPorterDuffXferProcessor.h b/include/gpu/effects/GrPorterDuffXferProcessor.h
index be014a6b04822387c191286f315001c69717f431..457c6eabc8c4f451d0935b6bc0b505863e6c5836 100644
--- a/include/gpu/effects/GrPorterDuffXferProcessor.h
+++ b/include/gpu/effects/GrPorterDuffXferProcessor.h
@@ -21,10 +21,16 @@ public:
     void getInvariantBlendedColor(const GrProcOptInfo& colorPOI,
                                   GrXPFactory::InvariantBlendedColor*) const override;
 
+
+    /** Because src-over is so common we special case it for performance reasons. If this returns
+        null then the SimpleSrcOverXP() below should be used. */
     static GrXferProcessor* CreateSrcOverXferProcessor(const GrCaps& caps,
                                                        const GrPipelineOptimizations& optimizations,
                                                        bool hasMixedSamples,
                                                        const GrXferProcessor::DstTexture*);
+    /** This XP implements non-LCD src-over using hw blend with no optimizations. It is returned
+        by reference because it is global and its ref-cnting methods are not thread safe. */
+    static const GrXferProcessor& SimpleSrcOverXP();
 
     static inline void SrcOverInvariantBlendedColor(
                                                 GrColor inputColor,