Support non-"fully active" documents

[anssiko]

We should check that the spec complies with https://w3ctag.github.io/design-principles/#support-non-fully-active

Given https://w3c.github.io/sensors/#can-expose-sensor-readings is conditional to https://html.spec.whatwg.org/multipage/interaction.html#currently-focused-area-of-a-top-level-browsing-context (among other checks) we may be good.

@marcoscaceres is probably interested in this design discussion.

[rakuco]

For the record: I've recently landed a patch in Chromium that makes Sensor.start() do nothing when the document is not fully active (strictly speaking, when there's no attached frame i.e. there's no browsing context). At least that part of the spec should be updated to take this into consideration at some point.