Using override keyword in ObjectVisitor class hierarchy.

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index c72effc08fb36ed83e1f16f24695f0175a0895b4..db4162ac57387450e9963fcc3776e80d2a1155b3 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -349,9 +349,9 @@ class IncrementalMarkingRootMarkingVisitor : public ObjectVisitor {
       IncrementalMarking* incremental_marking)
       : heap_(incremental_marking->heap()) {}
 
-  void VisitPointer(Object** p) { MarkObjectByPointer(p); }
+  void VisitPointer(Object** p) override { MarkObjectByPointer(p); }
 
-  void VisitPointers(Object** start, Object** end) {
+  void VisitPointers(Object** start, Object** end) override {
     for (Object** p = start; p < end; p++) MarkObjectByPointer(p);
   }