[soft navs] Integrates PaintTimingMixin into SoftNavigation entry.

The soft-navigation entry will now measure and report paint timings,
which is useful as the "first contentful paint" (FCP).

It also has a side benefit: by delaying until after presentation time is
measured, we delay emitting the performance entry to the performance
timeline. This gives Event Timing enough time to report entries (well,
for click and keyboard entries, which is all we support, for now).

This still isn't perfect: the current paint timing isn't necessarily the
"first contentful paint" in the same sense as hard-FCP:

- We delay measuring until after navigationID is incremented-- but this isn't guaranteed to happen in the "first" paint for the Context.  Today, our paint area requirement is so small that it is highly likely to be.  Still, we won't emit LCP until after this point, either.
- We use LCP semantics: ie fully loaded images, because we only check for navigations after "LCP candidates" arrive.  This has additional benefits such as actually checking if the content is visible, but it does delay the timing.  Perhaps a more proper description would be: Hard FCP is "first paint of the first animation frame with any contentful nodes", while Soft FCP is currently "first fully contentful paint" which is also equivalent to first LCP candidate.

In the future soft.FCP might no longer be first soft.LCP candidate, if
we record the first paint before navigation is emitted.

Bug: 424448145
Change-Id: I24d3f14c580b0c88e9c6416fe424d60e81b098c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6657746
Commit-Queue: Michal Mocny <mmocny@chromium.org>
Reviewed-by: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1476948}