[net] Add AsyncDNS.TTL histogram.

net/base/expiring_cache.h

 // Copyright (c) 2012 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 NET_BASE_EXPIRING_CACHE_H_
 #define NET_BASE_EXPIRING_CACHE_H_
 
 #include <map>
 #include <utility>
 
@@ skipping 102 matching lines @@
 
     // Immediately remove expired entries.
     if (!expiration_comp_(now, it->second.second)) {
       entries_.erase(it);
       return NULL;
     }
 
     return &it->second.first;
   }
 
-  // Updates or replaces the value associated with |key|.
-  void Put(const KeyType& key,
-           const ValueType& value,
-           const ExpirationType& now,
-           const ExpirationType& expiration) {
+  // Updates or replaces the value associated with |key|. Returns a pointer to
+  // the stored value.
+  ValueType* Put(const KeyType& key,
+                 const ValueType& value,
+                 const ExpirationType& now,
+                 const ExpirationType& expiration) {
     typename EntryMap::iterator it = entries_.find(key);
     if (it == entries_.end()) {
       // Compact the cache if it grew beyond the limit.
       if (entries_.size() == max_entries_ )
         Compact(now);
 
       // No existing entry. Creating a new one.
-      entries_.insert(std::make_pair(key, Entry(value, expiration)));
+      it = entries_.insert(std::make_pair(key, Entry(value, expiration))).first;
     } else {
       // Update an existing cache entry.
       it->second.first = value;
       it->second.second = expiration;
     }
+    return &it->second.first;
   }
 
   // Empties the cache.
   void Clear() {
     entries_.clear();
   }
 
   // Returns the number of entries in the cache.
   size_t size() const { return entries_.size(); }
 
@@ skipping 33 matching lines @@
 
   EntryMap entries_;
   ExpirationCompare expiration_comp_;
 
   DISALLOW_COPY_AND_ASSIGN(ExpiringCache);
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_EXPIRING_CACHE_H_