[soft navs] Delete performance_timeline_constants.h.

* Moving the SoftNavigationMetrics struct to
  web_performance_metrics_for_reporting.h.

* Moving the default value into navigation_id_generator.h,
  now as blink::kNavigationIdAbsentValue - this is more
  descriptive than previously, for the few places where
  it's actually useful; but for initializing the
  SoftNavigationMetrics struct I think a simple "= 0" is
  better / more consistent with the other fields / structs in
  web_performance_metrics_for_reporting.h.

* In the unittests, now using CreateSoftNavigationMetrics(),
  instead of repeating the initialization of the mojom struct.

This is a follow-up to crrev.com/c/6604387.

Bug: 421337393
Change-Id: I0db7164c0040b1e3d8ace1baab9a0936ce8eb6a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6731258
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1491120}