add missing externs to repo

externs/webextension_polyfill/runtime/OnRestartRequiredReason.hx

@@ -0,0 +1,8 @@
+package webextension_polyfill.runtime;
+
+/**
+	The reason that the event is being dispatched. 'app_update' is used when the restart is needed because the application
+	is updated to a newer version. 'os_update' is used when the restart is needed because the browser/OS is updated to a
+	newer version. 'periodic' is used when the system runs for more than the permitted uptime set in the enterprise policy.
+**/
+typedef OnRestartRequiredReason = String;