Enable SharedArrayBuffers with sequential threads?

https://bugzilla.mozilla.org/show_bug.cgi?id=1563335 implements a preference, dom.workers.serialized-sab-access, that, when flipped to true, allows using SharedArrayBuffers with some fingerprinting resistance (see: Mozilla's https://bugzilla.mozilla.org/show_bug.cgi?id=1617872 for binding that pref directly to the RFP mode).

We probably want to do that if we want to enable SharedArrayBuffers at all again.

This is planned to land on release in the Firefox 79/80 timeframe, see: https://groups.google.com/forum/#!topic/mozilla.dev.platform/-hYWoob95LI.