CHECK index in NavigationControllerImpl::GoToIndex

This should never have been called with invalid indices, and as of
crrev.com/c/4554558 we believe this is bounds checked by all callers.

If this still fails additional call sites should be updated to validate
indices as appropriate (perhaps with this change temporarily reverted).

The last known failure report was in 115.0.5748.0 and no reports in M113
(previous stable) have >100 instances reported. This seems reasonably
riskless to CHECK at this point.

Bug: 1411462
Change-Id: If3b49be90656210e28db5f8921baf3e490bea52a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4591009
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1154524}