Reduce the memory usage of bookmarks storage

components/bookmarks/browser/titled_url_index.h

Index: components/bookmarks/browser/titled_url_index.h
diff --git a/components/bookmarks/browser/titled_url_index.h b/components/bookmarks/browser/titled_url_index.h
index d6a0fbae8d33b6e3397103b6428422c9014d7b60..f6df5980d2e8a592f297c5bd71971f697f625d60 100644
--- a/components/bookmarks/browser/titled_url_index.h
+++ b/components/bookmarks/browser/titled_url_index.h
@@ -8,10 +8,10 @@
 #include <stddef.h>
 
 #include <map>
-#include <set>
 #include <string>
 #include <vector>
 
+#include "base/containers/flat_set.h"
 #include "base/macros.h"
 #include "base/strings/string16.h"
 #include "components/query_parser/query_parser.h"
@@ -50,7 +50,7 @@ class TitledUrlIndex {
 
  private:
   using TitledUrlNodes = std::vector<const TitledUrlNode*>;
-  using TitledUrlNodeSet = std::set<const TitledUrlNode*>;
+  using TitledUrlNodeSet = base::flat_set<const TitledUrlNode*>;
   using Index = std::map<base::string16, TitledUrlNodeSet>;
 
   // Constructs |sorted_nodes| by copying the matches in |matches| and sorting