base: Remove non-const refs from the MRUCache implementation.

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 195 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(MRUCacheBase);
 };
 
 // MRUCache --------------------------------------------------------------------
 
 // A functor that does nothing. Used by the MRUCache.
 template<class PayloadType>
 class MRUCacheNullDeletor {
  public:
-  void operator()(PayloadType& payload) {
-  }
+  void operator()(const PayloadType& payload) {}
 };
 
 // A container that does not do anything to free its data. Use this when storing
 // value types (as opposed to pointers) in the list.
 template <class KeyType, class PayloadType>
 class MRUCache : public MRUCacheBase<KeyType,
                                      PayloadType,
                                      MRUCacheNullDeletor<PayloadType> > {
  private:
   typedef MRUCacheBase<KeyType, PayloadType,
       MRUCacheNullDeletor<PayloadType> > ParentType;
 
  public:
   // See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.
   explicit MRUCache(typename ParentType::size_type max_size)
       : ParentType(max_size) {
   }
   virtual ~MRUCache() {
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MRUCache);
 };
 
 // OwningMRUCache --------------------------------------------------------------
 
 template<class PayloadType>
 class MRUCachePointerDeletor {
  public:
-  void operator()(PayloadType& payload) {
-    delete payload;
-  }
+  void operator()(const PayloadType& payload) { delete payload; }
 };
 
 // A cache that owns the payload type, which must be a non-const pointer type.
 // The pointers will be deleted when they are removed, replaced, or when the
 // cache is destroyed.
 template <class KeyType, class PayloadType>
 class OwningMRUCache
     : public MRUCacheBase<KeyType,
                           PayloadType,
                           MRUCachePointerDeletor<PayloadType> > {
@@ skipping 41 matching lines @@
   virtual ~HashingMRUCache() {
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(HashingMRUCache);
 };
 
 }  // namespace base
 
 #endif  // BASE_CONTAINERS_MRU_CACHE_H_