Reduce the memory usage of bookmarks storage

components/bookmarks/browser/titled_url_node_sorter.h

 // Copyright 2016 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.
 
 #ifndef COMPONENTS_BOOKMARKS_BROWSER_TITLED_URL_NODE_SORTER_H_
 #define COMPONENTS_BOOKMARKS_BROWSER_TITLED_URL_NODE_SORTER_H_
 
-#include <set>
 #include <vector>
 
+#include "base/containers/flat_set.h"
+
 namespace bookmarks {
 
 class TitledUrlNode;
 
 class TitledUrlNodeSorter {
  public:
   using TitledUrlNodes = std::vector<const TitledUrlNode*>;
-  using TitledUrlNodeSet = std::set<const TitledUrlNode*>;
+  using TitledUrlNodeSet = base::flat_set<const TitledUrlNode*>;
 
   virtual ~TitledUrlNodeSorter() {};
 
   // Sorts |matches| in an implementation-specific way, placing the results in
   // |sorted_nodes|.
   virtual void SortMatches(const TitledUrlNodeSet& matches,
                            TitledUrlNodes* sorted_nodes) const = 0;
 };
 
 }  // namespace bookmarks
 
 #endif  // COMPONENTS_BOOKMARKS_BROWSER_TITLED_URL_NODE_SORTER_H_