Create a static instances of SrcOver XferProcessor

src/gpu/effects/GrCoverageSetOpXP.cpp

Index: src/gpu/effects/GrCoverageSetOpXP.cpp
diff --git a/src/gpu/effects/GrCoverageSetOpXP.cpp b/src/gpu/effects/GrCoverageSetOpXP.cpp
index bcdb6d81dcf89c2838744e18af09240709ff0f58..95c4a88feb77b3f4eb4392cb044e30759e6626cd 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.cpp
+++ b/src/gpu/effects/GrCoverageSetOpXP.cpp
@@ -36,7 +36,7 @@ private:
                                                  const GrProcOptInfo& coveragePOI,
                                                  bool doesStencilWrite,
                                                  GrColor* color,
-                                                 const GrCaps& caps) override;
+                                                 const GrCaps& caps) const override;
 
     void onGetGLSLProcessorKey(const GrGLSLCaps& caps, GrProcessorKeyBuilder* b) const override;
 
@@ -111,7 +111,7 @@ CoverageSetOpXP::onGetOptimizations(const GrProcOptInfo& colorPOI,
                                     const GrProcOptInfo& coveragePOI,
                                     bool doesStencilWrite,
                                     GrColor* color,
-                                    const GrCaps& caps) {
+                                    const GrCaps& caps) const {
     // We never look at the color input
     return GrXferProcessor::kIgnoreColor_OptFlag; 
 }