Using override keyword in ObjectVisitor class hierarchy.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 9e205caf81092b1382d18adc47cfe0ab26ab1a36..2966bcfe2e03a107757b441ecd28888e2b9b53ec 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -2429,14 +2429,14 @@ class AlwaysAllocateScope {
 // objects in a heap space but above the allocation pointer.
 class VerifyPointersVisitor : public ObjectVisitor {
  public:
-  inline void VisitPointers(Object** start, Object** end);
+  inline void VisitPointers(Object** start, Object** end) override;
 };
 
 
 // Verify that all objects are Smis.
 class VerifySmisVisitor : public ObjectVisitor {
  public:
-  inline void VisitPointers(Object** start, Object** end);
+  inline void VisitPointers(Object** start, Object** end) override;
 };
 
 
@@ -2697,7 +2697,7 @@ class PathTracer : public ObjectVisitor {
         object_stack_(20),
         no_allocation() {}
 
-  virtual void VisitPointers(Object** start, Object** end);
+  void VisitPointers(Object** start, Object** end) override;
 
   void Reset();
   void TracePathFrom(Object** root);