Using override keyword in ObjectVisitor class hierarchy.

src/heap/scavenger.h

Index: src/heap/scavenger.h
diff --git a/src/heap/scavenger.h b/src/heap/scavenger.h
index 44da98c86c73462e4dd579bd925eb4f984733698..b180879db2dd3be3124c903c776ccf0532646503 100644
--- a/src/heap/scavenger.h
+++ b/src/heap/scavenger.h
@@ -48,8 +48,8 @@ class ScavengeVisitor : public ObjectVisitor {
  public:
   explicit ScavengeVisitor(Heap* heap) : heap_(heap) {}
 
-  void VisitPointer(Object** p);
-  void VisitPointers(Object** start, Object** end);
+  void VisitPointer(Object** p) override;
+  void VisitPointers(Object** start, Object** end) override;
 
  private:
   inline void ScavengePointer(Object** p);