[Client Hints] Implement criticalCHRestart timing field

This timestamp allows developers to understand the cost of a Critical-CH
HTTP Header related restart in the load of a page. This is behind an
experimental realtime feature and tested in WPTs. The ongoing launch is:
https://groups.google.com/a/chromium.org/g/blink-dev/c/f28WWMD8HVE/

Bug: 1425910
Change-Id: I68e3a6b9d37f3e11cb3b27469321106940aa1246
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4538244


Auto-Submit: Ari Chivukula <arichiv@chromium.org>
Reviewed-by: Nick Harper <nharper@chromium.org>
Commit-Queue: Nick Harper <nharper@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146593}