Compiler error

src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper_fallback.cc

@@ -31,7 +31,7 @@ static grpc_ares_request* grpc_dns_lookup_ares_locked_impl(
     grpc_pollset_set* interested_parties, grpc_closure* on_done,
     std::unique_ptr<grpc_core::ServerAddressList>* addrs, bool check_grpclb,
     char** service_config_json, int query_timeout_ms,
-    std::shared_ptr<WorkSerializer> work_serializer) {
+    std::shared_ptr<grpc_core::WorkSerializer> work_serializer) {
   return NULL;
 }
 
@@ -40,7 +40,7 @@ grpc_ares_request* (*grpc_dns_lookup_ares_locked)(
     grpc_pollset_set* interested_parties, grpc_closure* on_done,
     std::unique_ptr<grpc_core::ServerAddressList>* addrs, bool check_grpclb,
     char** service_config_json, int query_timeout_ms,
-    std::shared_ptr<WorkSerializer> work_serializer) =
+    std::shared_ptr<grpc_core::WorkSerializer> work_serializer) =
     grpc_dns_lookup_ares_locked_impl;
 
 static void grpc_cancel_ares_request_locked_impl(grpc_ares_request* r) {}