This code sloppily extends the existing
(pid annotation to add subframe information.
I'm also open to an alternate mechanism for communicating this information. Apparently this hover tooltip is not accessible to screenreaders, so it may be worthwhile to add an alternate mechanism for that.
In the future it may be worthwhile to add info about the PID to a support page, like
about:support. As with fission we use one process per site, it would be possible to correlate iframes to PIDs by comparing the site loaded in them, and looking up the corresponding PID.
Finally, it may also be worthwhile to introduce a nightly-only WebExtension API which is able to read info about which PID is being used for a subframe, which can be then used to provide more flexible out-of-band information.