Fix compilation issue with base::flat_map.

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 8ae5a50e357fcc9615dab6171037b1aedef0a969..e2cb003338c5fb8171487b61a3676dc0cf7c7fa5 100644
--- a/base/containers/flat_map.h
+++ b/base/containers/flat_map.h
@@ -240,7 +240,7 @@ 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;
 }