Ensure there is always an active tab in nonempty TabStripModels.

- CHECK (NotFatalUntil::M124 to prevent stability issues) this invariant
whenever the selection is changed or tabs are added/removed.
- Activate the first tab added to an empty tabstrip, even if that is not
requested by `add_types`.
- Update tests accordingly, and add a new one covering the first-tab
activation behavior.
- Fix a crash in tab dragging introduced by activating the first tab
added to an empty tabstrip.

The tab dragging crash occurred when dragging an initially collapsed
group out into a new window. In that case, the group is created
collapsed in the new window, and immediately expanded upon adding the
first, active tab. However, the order of notifications in the
TabStripModel is such that the group isn't yet created in the TabStrip
when it goes to expand it; this of course crashes. I sidestep this
problem by creating the new group already expanded, but the more
fundamental preexisting issue in how TabStripModel and TabStrip
communicate during group creation is out of scope for this CL.

Bug: 1496583
Change-Id: I58b70dc43424e5d3a3dc76215813d8355d70ff0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5166847
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1243173}