Add trace event for debugging scrollbar existence.

Bug: 1355780
Change-Id: If62ecce0f41dbcd42b3cd1beebf992743c618f9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4304460


Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Auto-Submit: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1112863}