Enable paint entry buffering for soft navigations

Previously, paint timing entries were not being properly buffered, even
if they were fired when observed.
This CL fixes that, adds tests to that effect and ensures that the
buffering limits are respected and the dropped entries counter is being
incremented.

Bug: 1397720
Change-Id: Id53f0ea19fd85895b7a1942eb8faff77c7a5c49b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4084861
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1080420}