[FPS] Use NetLog API unconditionally and more idiomatically

I took a look at the events in the netlog viewer (after finding
https://chromium.googlesource.com/chromium/src/+/HEAD/net/docs/crash-course-in-net-internals.md),
and learned there were better APIs for handling begin/end parts of a
single event. It also made it clear that the FPS metadata event didn't
need to include the request URL, since that's already recorded by the
parent URL_REQUEST_START_JOB event.

This CL also modifies the timing of the events so that they include
the round-trip for getting the cache filter info, since that is
unconditional.

Change-Id: Idd28b2645a4cf2c85f95933b50b7057abf12e190
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4832491
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1191351}