Implement pref_value_map swap function.

chrome/browser/prefs/pref_value_map.h

 // Copyright (c) 2011 The Chromium 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 CHROME_BROWSER_PREFS_PREF_VALUE_MAP_H_
 #define CHROME_BROWSER_PREFS_PREF_VALUE_MAP_H_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 24 matching lines @@
   // non-NULL. Returns true if the value changed.
   bool SetValue(const std::string& key, base::Value* value);
 
   // Removes the value for |key| from the map. Returns true if a value was
   // removed.
   bool RemoveValue(const std::string& key);
 
   // Clears the map.
   void Clear();
 
+  // Swaps the contents of two maps.
+  void Swap(PrefValueMap* other);
+
   iterator begin();
   iterator end();
   const_iterator begin() const;
   const_iterator end() const;
 
   // Gets a boolean value for |key| and stores it in |value|. Returns true if
   // the value was found and of the proper type.
   bool GetBoolean(const std::string& key, bool* value) const;
 
   // Sets the value for |key| to the boolean |value|.
@@ skipping 21 matching lines @@
 
  private:
   typedef std::map<std::string, base::Value*> Map;
 
   Map prefs_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefValueMap);
 };
 
 #endif  // CHROME_BROWSER_PREFS_PREF_VALUE_MAP_H_