It's a bit more complicated. Chrome has had the ability to run separate tabs in separate processes since day 1. However, quite often, JavaScript from separate web sites would end up running in the same process. Specifically, (i)frames, popup windows, and sometimes tabs created by a site would run in the same process as the creating site, even if the child window displayed a completely different domain. In this case, in fact, the sites would run in the same V8 isolate. The only separation was "context" separation, which essentially means that the objects from one site were prohibited from accessing the objects from another even though they occupied the same heap.

More recently, Chrome has introduced Site Isolation, which actually allows iframes to run in separate processes. On desktop, this is enabled by default as of Chrome 67. On Android, it is not enabled yet, because it has been shown to be too expensive. The Chrome team wants to fix this, but it's not clear (to me, at least) whether they'll be able to overcome the inherent barriers here.

Chrome started working on Site Isolation before Spectre, but Spectre accelerated interest in it. My take is that Spectre is probably not the main reason that the Chrome security people (who are awesome, BTW) want to do it, but it provided a great excuse to rally support behind it.

For Workers -- much like for Android Chrome -- process-per-tenant is still too costly to be feasible. So, we need to pursue other approaches, as I've described elsewhere in this thread.

Thank you for the very detailed reply, that was an interesting read. Much appreciated!