Specify "time origin" computation

The time origin, which is the epoch for all performance timeline APIs, was previously defined in https://w3c.github.io/hr-time/#dfn-time-origin.

This makes its computation explicit, without changing its current definition and behavior. Notably:

* "time origin becomes an environment settings object algorithm, computed differently for windows vs. worker scopes.

* The 4 places where this timestamp is set are creation of an about:blank browsing context, the beginning of navigation, the confirmation of an unload prompt, and the creation of a worker.

* The timestamp is a "shared monotonic clock" value, which is defined in HR-TIME (and internally in ECMAScript).

As part of this PR, also refactored unload prompts to return a result rather than rely on a somewhat hand-wavy "refused to allow" property.

See w3c/hr-time#131 and w3c/navigation-timing#166. See also w3c/ServiceWorker#1621.