Make innerWindowID columns optional
The FrameTable's innerWindowID column is currently required. However it is quite useless for non-Firefox profiles and takes up a lot of space. It would be nice to make it optional.
┆Issue is synchronized with this Jira Task
At one point we tended to make things required so that it would be easier to manage in our codebase. But this is clearly a roadblock for the usage of the profile format outside of Firefox.
I wonder if we could have an external processed format (with optional fields) and an internal processed format (with the optional fields replaced at loading time with sane defaults)...
Once we remove the active tab view (#4997), we can just remove this column entirely, see #5443.