Add a few missing overrides found by a new clang warning.

src/serialize.cc

 // Copyright 2012 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.
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/base/platform/platform.h"
 #include "src/bootstrapper.h"
@@ skipping 734 matching lines @@
 
 
 // Used to insert a deserialized internalized string into the string table.
 class StringTableInsertionKey : public HashTableKey {
  public:
   explicit StringTableInsertionKey(String* string)
       : string_(string), hash_(HashForObject(string)) {
     DCHECK(string->IsInternalizedString());
   }
 
-  virtual bool IsMatch(Object* string) {
+  virtual bool IsMatch(Object* string) OVERRIDE {
     // We know that all entries in a hash table had their hash keys created.
     // Use that knowledge to have fast failure.
     if (hash_ != HashForObject(string)) return false;
     // We want to compare the content of two internalized strings here.
     return string_->SlowEquals(String::cast(string));
   }
 
   virtual uint32_t Hash() OVERRIDE { return hash_; }
 
   virtual uint32_t HashForObject(Object* key) OVERRIDE {
@@ skipping 1558 matching lines @@
 
 int SerializedCodeData::CheckSum(String* string) {
   int checksum = Version::Hash();
 #ifdef DEBUG
   uint32_t seed = static_cast<uint32_t>(checksum);
   checksum = static_cast<int>(IteratingStringHasher::Hash(string, seed));
 #endif  // DEBUG
   return checksum;
 }
 } }  // namespace v8::internal