[anchor]: Add a use counter for abspos fallbacks.

Since the behavior of abspos in a scroller may change with the update,
this patch adds a use counter to see how often abspos switches
position-try-fallbacks as an upper bound of breakage.

R=ikilpatrick@chromium.org, mstensho@chromium.org

Bug: 438515315
Change-Id: If4923844ded34b4c44ff68efc1440ec819983b44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6896868
Commit-Queue: Vladimir Levin <vmpstr@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Auto-Submit: Vladimir Levin <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1509878}