Add range insertion for base::flat_tree

base/containers/flat_map.h

Index: base/containers/flat_map.h
diff --git a/base/containers/flat_map.h b/base/containers/flat_map.h
index 225052b513ec6b3f2d18ea5bf6a7cd3a08651eec..ce50dcd736046851016029011e19542c7ae6f891 100644
--- a/base/containers/flat_map.h
+++ b/base/containers/flat_map.h
@@ -98,6 +98,8 @@ struct GetKeyFromValuePairFirst {
 //   Mapped&              operator[](Key&&);
 //   pair<iterator, bool> insert(const pair<Key, Mapped>&);
 //   pair<iterator, bool> insert(pair<Key, Mapped>&&);
+//   void                 insert(InputIterator first, InputIterator last,
+//                               FlatContainerDupes);
 //   pair<iterator, bool> emplace(Args&&...);
 //   iterator             emplace_hint(const_iterator, Args&&...);
 //