Use dst copies in porter duffer XP to correctly render certain blends.

BUG=skia:

Review URL: https://codereview.chromium.org/914003003
diff --git a/src/gpu/effects/GrCoverageSetOpXP.cpp b/src/gpu/effects/GrCoverageSetOpXP.cpp
index f0ec1f9..40998d5 100644
--- a/src/gpu/effects/GrCoverageSetOpXP.cpp
+++ b/src/gpu/effects/GrCoverageSetOpXP.cpp
@@ -228,7 +228,8 @@
 }
 
 GrXferProcessor*
-GrCoverageSetOpXPFactory::onCreateXferProcessor(const GrProcOptInfo& colorPOI,
+GrCoverageSetOpXPFactory::onCreateXferProcessor(const GrDrawTargetCaps& caps,
+                                                const GrProcOptInfo& colorPOI,
                                                 const GrProcOptInfo& covPOI,
                                                 const GrDeviceCoordTexture* dstCopy) const {
     return CoverageSetOpXP::Create(fRegionOp, fInvertCoverage);