Using override keyword in ObjectVisitor class hierarchy.

src/context-measure.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_CONTEXT_MEASURE_H_
 #define V8_CONTEXT_MEASURE_H_
 
 #include "src/address-map.h"
 #include "src/assert-scope.h"
 #include "src/objects.h"
 
 namespace v8 {
 namespace internal {
 
 class ContextMeasure : public ObjectVisitor {
  public:
   explicit ContextMeasure(Context* context);
 
   int Size() { return size_; }
   int Count() { return count_; }
 
-  void VisitPointers(Object** start, Object** end);
+  void VisitPointers(Object** start, Object** end) override;
 
  private:
   void MeasureObject(HeapObject* object);
   void MeasureDeferredObjects();
   void MeasureAndRecurse(HeapObject* object);
   bool IsShared(HeapObject* object);
 
   Context* context_;
 
   BackReferenceMap back_reference_map_;
   RootIndexMap root_index_map_;
 
   static const int kMaxRecursion = 16;
   int recursion_depth_;
   List<HeapObject*> deferred_objects_;
 
   int count_;
   int size_;
 
   DisallowHeapAllocation no_gc_;
 
   DISALLOW_COPY_AND_ASSIGN(ContextMeasure);
 };
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_CONTEXT_MEASURE_H_