Make innerWindowID columns optional

[mstange]

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

[julienw]

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)...

[mstange]

Once we remove the active tab view (#4997), we can just remove this column entirely, see #5443.