Add discardable memory emulation for non-android/mac platforms

base/containers/mru_cache.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.
 
 // This file contains a template for a Most Recently Used cache that allows
 // constant-time access to items using a key, but easy identification of the
 // least-recently-used items for removal.  Each key can only be associated with
 // one payload item at a time.
 //
 // The key object will be stored twice, so it should support efficient copying.
@@ skipping 105 matching lines @@
       return end();
     typename PayloadList::iterator iter = index_iter->second;
 
     // Move the touched item to the front of the recency ordering.
     ordering_.splice(ordering_.begin(), ordering_, iter);
     return ordering_.begin();
   }
 
   // Retrieves the payload associated with a given key and returns it via
   // result without affecting the ordering (unlike Get).
-  //
-  // TODO(brettw) We may want a const version of this function in the future.
   iterator Peek(const KeyType& key) {
     typename KeyIndex::const_iterator index_iter = index_.find(key);
     if (index_iter == index_.end())
       return end();
     return index_iter->second;
   }
 
+  const_iterator Peek(const KeyType& key) const {
+    typename KeyIndex::const_iterator index_iter = index_.find(key);
+    if (index_iter == index_.end())
+      return end();
+    return index_iter->second;
+  }
+
   // Erases the item referenced by the given iterator. An iterator to the item
   // following it will be returned. The iterator must be valid.
   iterator Erase(iterator pos) {
     deletor_(pos->second);
     index_.erase(pos->first);
     return ordering_.erase(pos);
   }
 
   // MRUCache entries are often processed in reverse order, so we add this
   // convenience function (not typically defined by STL containers).
@@ skipping 151 matching lines @@
   virtual ~HashingMRUCache() {
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(HashingMRUCache);
 };
 
 }  // namespace base
 
 #endif  // BASE_CONTAINERS_MRU_CACHE_H_