Paint timing: revert back to swap-time

This CL is a minimal change to use swap time instead of presentation
time in PageLoad.PaintTiming.NavigationToFirstContentfulPaint. This is a
tentative fix for the unexpected regressions in the metric. The change
is minimal since it may require a merge to Stable. A followup change
will record the presentation timestamp, or the delta with the swap
timestamp, in a separate metric.

Bug: 870707
Change-Id: I1b48968d010bbd6120a6fbac59e26dc789d23394
Reviewed-on: https://chromium-review.googlesource.com/1244371
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594331}
1 file changed