the version handshake; compatible is false when the worker speaks another protocol
tells the worker how many cross-origin resources the page loaded, for view id, and from which origins
Optionalorigins: Record<string, number>asks the worker what it holds
closes db when the page is hidden, so the page stays eligible for the back/forward cache
the id of this page's view, which the worker made when it handled the navigation; null if it made none
Optionalpath: string
the page's handle on the worker