[net] Add AsyncDNS.TTL histogram.

net/base/expiring_cache.h

Index: net/base/expiring_cache.h
diff --git a/net/base/expiring_cache.h b/net/base/expiring_cache.h
index fe2fc65a0d68d2f6b58da4ba5f06c2f37dd3a666..3cdf42deddae7e6c22d34f2b2e48d8e70deeaad9 100644
--- a/net/base/expiring_cache.h
+++ b/net/base/expiring_cache.h
@@ -120,11 +120,12 @@ class ExpiringCache {
     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.
@@ -132,12 +133,13 @@ class ExpiringCache {
         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.