added support for PLS path rendering

BUG=skia:3555
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1541903002

Committed: https://skia.googlesource.com/skia/+/7df3f5e127f8016d17b637cc48a6a4718f1a6822

Review URL: https://codereview.chromium.org/1541903002
diff --git a/src/gpu/GrXferProcessor.cpp b/src/gpu/GrXferProcessor.cpp
index b07b972..01534a3 100644
--- a/src/gpu/GrXferProcessor.cpp
+++ b/src/gpu/GrXferProcessor.cpp
@@ -217,3 +217,10 @@
     return (this->willReadDstColor(caps, optimizations, hasMixedSamples) &&
             !caps.shaderCaps()->dstReadInShaderSupport());
 }
+
+bool GrXPFactory::willReadDstColor(const GrCaps& caps,
+                                   const GrPipelineOptimizations& optimizations,
+                                   bool hasMixedSamples) const {
+    return optimizations.fOverrides.fUsePLSDstRead || this->onWillReadDstColor(caps, optimizations,
+                                                                               hasMixedSamples);
+}