Report pagination indicator event only when it is actually shown

Bug: 1427708
Change-Id: I2ca528f90891dd4d102dfbf3ff0626fcee280c15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4548323


Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Jian Li <jianli@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147335}