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

base/containers/mru_cache.h

Index: base/containers/mru_cache.h
diff --git a/base/containers/mru_cache.h b/base/containers/mru_cache.h
index 15ea2fccdc4104dfbae4961cc24674c475230771..ed1ad251a2f607ee0b2a8f0384d9368474f4cf54 100644
--- a/base/containers/mru_cache.h
+++ b/base/containers/mru_cache.h
@@ -213,8 +213,7 @@ class MRUCacheBase {
 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
@@ -244,9 +243,7 @@ class MRUCache : public MRUCacheBase<KeyType,
 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.