Reland "[iOS] Adding methods to show/hide SignInPromo on the FeedTopSectionVC"

This reverts commit a10618b8a96579eb400865d1d0e473bbba99fb93.

Reason for revert: Relanding with solution for crash. The CHECK can
be removed as the flag was originally used for an experiment, and soon
the Top Section will support multiple Promos.

Original change's description:
> Revert "[iOS] Adding methods to show/hide SignInPromo on the FeedTopSectionVC"
>
> This reverts commit e457ebf58fbaa8e76bf9dc7c72ec84fdc4427d61.
>
> Reason for revert: Crashes Canary. Reverting until further investigation.
>
> Original change's description:
> > [iOS] Adding methods to show/hide SignInPromo on the FeedTopSectionVC
> >
> > Added two new methods to show and hide the SignInPromo on the
> > FeedTopSectionViewController that can be called directly to change the
> > visibility of the promo in question.
> >
> > Change-Id: I23d4dd34d09216f460b18d539b36f45befe607ce
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4887842
> > Auto-Submit: Guillem Perez <guiperez@google.com>
> > Reviewed-by: Adam Arcaro <adamta@google.com>
> > Commit-Queue: Guillem Perez <guiperez@google.com>
> > Cr-Commit-Position: refs/heads/main@{#1209724}
>
> Change-Id: Ibeea5a1a7f6fa57624aba3869119d6d9ba4ab233
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4940890
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Auto-Submit: Guillem Perez <guiperez@google.com>
> Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#1210168}

Change-Id: If0c5bdc22c46842ecc4c78d31ed6e62786c53b7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4944190
Auto-Submit: Guillem Perez <guiperez@google.com>
Commit-Queue: Guillem Perez <guiperez@google.com>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210432}