Make XPFragmentBuilder only Builder with access to DstCopy.

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 0c4818431a4b38b64061a056e020c616a79fea61..69b9ea417e23e473e1df4e63acd06db1e1f6f05c 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.cpp
+++ b/src/gpu/effects/GrCoverageSetOpXP.cpp
@@ -74,7 +74,7 @@ public:
 private:
     void onEmitCode(const EmitArgs& args) override {
         const CoverageSetOpXP& xp = args.fXP.cast<CoverageSetOpXP>();
-        GrGLFPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+        GrGLXPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
 
         if (xp.invertCoverage()) {
             fsBuilder->codeAppendf("%s = 1.0 - %s;", args.fOutputPrimary, args.fInputCoverage);