Previously the POSIX version of TimeTicks::Now() was used on Fuchsia.
It's more efficient to call zx_clock_get_monotonic() directly.

Bug: chromium:1317914
Change-Id: I56da954a2567bcb866100c157878176bcb7cf319
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3595601
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80224}