Add discardable memory emulation for non-android/mac platforms

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 e59e909839165e9fd9cd1b1f106e35736ecd01a5..15ea2fccdc4104dfbae4961cc24674c475230771 100644
--- a/base/containers/mru_cache.h
+++ b/base/containers/mru_cache.h
@@ -123,8 +123,6 @@ class MRUCacheBase {
 
   // 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())
@@ -132,6 +130,13 @@ class MRUCacheBase {
     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) {