[iOS] Fix tab count for VoiceOver

The count wasn't updated when new tabs were added.

Fixed: 330856010
Change-Id: Ib215c7eb5dd2044fe25c3daa9993b6699bcbfa09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5380092


Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Ewann Pellé <ewannpv@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1276831}