1. periodicsyncevent api

2. periodicsyncevent api: `periodicsyncevent()` constructor

3. periodicsyncevent api: tag

4. periodicsyncmanager api

5. periodicsyncmanager api: gettags

6. periodicsyncmanager api: register

7. periodicsyncmanager api: unregister

8. serviceworkerglobalscope api: `periodicsync` event

9. serviceworkerregistration api: periodicsync