[chromeos] Use base::TimeTicks in BootTime metrics reporting.

BootTime metrics historically used base::Time as timestamp (double
converting from base::TimeTicks) which results in precision loss and events reordering in traces.

We should use base::TimeTicks and avoid double conversion.

Bug: b:261503160
Change-Id: Iadd21bf02de7ccc786a4d09770d05a9d8523c689
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4080911
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Auto-Submit: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1080009}