Using override keyword in ObjectVisitor class hierarchy.

test/cctest/test-global-handles.cc

Index: test/cctest/test-global-handles.cc
diff --git a/test/cctest/test-global-handles.cc b/test/cctest/test-global-handles.cc
index 20aaeac73eea5d10a1d7dadc31e179aaebe36a9c..cc95df21cf7f763c784ee8f85cbfb1a69c4abc7d 100644
--- a/test/cctest/test-global-handles.cc
+++ b/test/cctest/test-global-handles.cc
@@ -75,7 +75,7 @@ class TestRetainedObjectInfo : public v8::RetainedObjectInfo {
 
 class TestObjectVisitor : public ObjectVisitor {
  public:
-  virtual void VisitPointers(Object** start, Object** end) {
+  void VisitPointers(Object** start, Object** end) override {
     for (Object** o = start; o != end; ++o)
       visited.Add(*o);
   }