Fix compilation issue with base::flat_map.

base/containers/flat_map.h

 // Copyright 2017 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 BASE_CONTAINERS_FLAT_MAP_H_
 #define BASE_CONTAINERS_FLAT_MAP_H_
 
 #include <utility>
 
 #include "base/containers/flat_tree.h"
@@ skipping 222 matching lines @@
   return *this;
 }
 
 // ----------------------------------------------------------------------------
 // Insert operations.
 
 template <class Key, class Mapped, class Compare>
 auto flat_map<Key, Mapped, Compare>::operator[](const Key& key) -> Mapped& {
   typename tree::iterator found = tree::lower_bound(key);
   if (found == tree::end() || tree::key_comp()(key, found->first))
-    found = unsafe_emplace(found, key, Mapped());
+    found = tree::unsafe_emplace(found, key, Mapped());
   return found->second;
 }
 
 template <class Key, class Mapped, class Compare>
 auto flat_map<Key, Mapped, Compare>::operator[](Key&& key) -> Mapped& {
   const Key& key_ref = key;
   typename tree::iterator found = tree::lower_bound(key_ref);
   if (found == tree::end() || tree::key_comp()(key, found->first))
     found = tree::unsafe_emplace(found, std::move(key), Mapped());
   return found->second;
 }
 
 // ----------------------------------------------------------------------------
 // General operations.
 
 template <class Key, class Mapped, class Compare>
 void flat_map<Key, Mapped, Compare>::swap(flat_map& other) {
   tree::swap(other);
 }
 
 }  // namespace base
 
 #endif  // BASE_CONTAINERS_FLAT_MAP_H_