[iOS] Fix TabGroup back button

When the user long press on back, they can directly pop the item.
We need to consider the didPopItem in addition of returning no for
shouldPopItem.

Fixed: 331907929
Change-Id: I9282d33970eae3bbe28e7e5a4dc034031d831d15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401965


Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Commit-Queue: Aliona Dangla <alionadangla@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280146}