[ios] Only record SetUpList item shown metric for items that are shown

This change is necessary because with the SetUpList in the MagicStack,
we only show at most two items in the compact SetUpList.

Change-Id: I798523f7ea8c604058c5fcfe63dcb1de44c5ad0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5443504
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Scott Yoder <scottyoder@google.com>
Cr-Commit-Position: refs/heads/main@{#1285762}