Allow custom comparators for the MRUCache

//base's MRUCache allowed specifying key and value types, like
an underlying std::map<>, but the default base::MRUCache explicitly
specialized the comparator as std::less<>. Rather than require the
caller to overload std::less<> or operator<, allow the comparator
to be specified as a templated argument.

BUG=665735

Review-Url: https://codereview.chromium.org/2532313002
Cr-Commit-Position: refs/heads/master@{#434850}

base/containers/mru_cache.h

@@ -209,10 +209,12 @@ class MRUCacheBase {
 
 // 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, std::less<KeyType>> {
+template <class KeyType,
+          class PayloadType,
+          class CompareType = std::less<KeyType>>
+class MRUCache : public MRUCacheBase<KeyType, PayloadType, CompareType> {
  private:
-  using ParentType = MRUCacheBase<KeyType, PayloadType, std::less<KeyType>>;
+  using ParentType = MRUCacheBase<KeyType, PayloadType, CompareType>;
 
  public:
   // See MRUCacheBase, noting the possibility of using NO_AUTO_EVICT.