Create a static instances of SrcOver XferProcessor

src/effects/SkArithmeticMode_gpu.cpp

Index: src/effects/SkArithmeticMode_gpu.cpp
diff --git a/src/effects/SkArithmeticMode_gpu.cpp b/src/effects/SkArithmeticMode_gpu.cpp
index 4d74082fb349b8bf262624bcd7a35d0f28d47738..95bb10d7ac2a49a87de32291929dcf1065fc93be 100644
--- a/src/effects/SkArithmeticMode_gpu.cpp
+++ b/src/effects/SkArithmeticMode_gpu.cpp
@@ -171,7 +171,7 @@ private:
                                                  const GrProcOptInfo& coveragePOI,
                                                  bool doesStencilWrite,
                                                  GrColor* overrideColor,
-                                                 const GrCaps& caps) override;
+                                                 const GrCaps& caps) const override;
 
     void onGetGLSLProcessorKey(const GrGLSLCaps& caps, GrProcessorKeyBuilder* b) const override;
 
@@ -261,7 +261,7 @@ GrXferProcessor::OptFlags ArithmeticXP::onGetOptimizations(const GrProcOptInfo&
                                                            const GrProcOptInfo& coveragePOI,
                                                            bool doesStencilWrite,
                                                            GrColor* overrideColor,
-                                                           const GrCaps& caps) {
+                                                           const GrCaps& caps) const {
    return GrXferProcessor::kNone_OptFlags;
 }