[PWA/Nav Capture] Break up latency metrics based on navigation starting

The initial latency metrics added to measure the time it took for the
browser to enqueue launch params are now being broken up into whether
this was done as a result of a new navigation vs whether this was done
because an existing container was found with no new navigation
happening.

This is useful to measure the latency differences between apps that
have a launch handling mode of focus-existing vs other modes. The
existing histogram has been renamed because it has not yet started
collecting any metrics:

https://uma.googleplex.com/p/chrome/timeline_v2?sid=3f5f8e1a65128468b5e5ac514dc926c9

Fixed: 409792072
Change-Id: I588c8314ede1e80790fde0576d9311772dcb5d9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6524766
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: May Siem <msiem@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Dibyajyoti Pal <dibyapal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1458177}