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/GrGpu.h b/src/gpu/GrGpu.h
index bfcaeae..2140349 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -349,6 +349,9 @@
 
     void draw(const DrawArgs&, const GrVertices&);
 
+    // Called by drawtarget when flushing. Provides a hook for working around an ARM PLS driver bug.
+    virtual void performFlushWorkaround();
+
     ///////////////////////////////////////////////////////////////////////////
     // Debugging and Stats