Create a static instances of SrcOver XferProcessor

src/gpu/effects/GrDisableColorXP.cpp

Index: src/gpu/effects/GrDisableColorXP.cpp
diff --git a/src/gpu/effects/GrDisableColorXP.cpp b/src/gpu/effects/GrDisableColorXP.cpp
index abeaa8c1cc888d44657928ab99f78142f235bbfc..e669f8b0240a12bb9983b296c2f3b772b7a0adc2 100644
--- a/src/gpu/effects/GrDisableColorXP.cpp
+++ b/src/gpu/effects/GrDisableColorXP.cpp
@@ -32,7 +32,7 @@ private:
     GrXferProcessor::OptFlags onGetOptimizations(const GrPipelineOptimizations& optimizations,
                                                  bool doesStencilWrite,
                                                  GrColor* color,
-                                                 const GrCaps& caps) override {
+                                                 const GrCaps& caps) const override {
         return GrXferProcessor::kIgnoreColor_OptFlag | GrXferProcessor::kIgnoreCoverage_OptFlag;
     }