Create new metrics to record CLS UKM metrics at the first OnHidden

Right now, we report the CLS UKM metric at OnComplete, which may not be
reported when the tabs are never closed or the browser crashes. Thus, we
get around 5% less of CLS reported than FCP. Since we cannot record
twice on the metrics in the singular event, we need to create new CLS
metrics that will record the layout shift score until the tab first
becomes backgrounded from foregrounded, which may complement the layout
shift values which are not able to be recorded in current CLS metrics.

The privacy review of adding these experimental CLS metrics:
https://docs.google.com/document/d/1tyGBPlqHL81t-YeyAGoJ9YkkPj8G0cbIP8ZGd3QAWHI/

Bug: 1210681
Change-Id: Iba1be23c50dc9a008752d9bd6f5a2c0d73772b28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3251379
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/main@{#945168}