Convert child FPs from refs to pending executions when parent converts

Review URL: https://codereview.chromium.org/1315923004
diff --git a/src/gpu/GrPrimitiveProcessor.h b/src/gpu/GrPrimitiveProcessor.h
index bd769e4..f6ad6b1 100644
--- a/src/gpu/GrPrimitiveProcessor.h
+++ b/src/gpu/GrPrimitiveProcessor.h
@@ -264,6 +264,7 @@
     size_t fVertexStride;
 
 private:
+    void notifyRefCntIsZero() const final {};
     virtual bool hasExplicitLocalCoords() const = 0;
 
     bool fIsPathRendering;