The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/56e6ce24e659c3ae732ecdfb36e25f9c02da5057

commit 56e6ce24e659c3ae732ecdfb36e25f9c02da5057
Author: Makoto Shimazu <shimazu@chromium.org>
Date: Wed Jun 20 15:41:39 2018

Add PageLoad.Clients.ServiceWorker.PaintTiming.NavigationToFirstPaint

PaintTiming.NavigationToFirstPaint is one of key loading metrics, and we'd add
it before starting ServceWorkerServicification (a.k.a NetS13nServiceWorker)
experiement.

Bug:  852698 
Change-Id: I2d7b7ad9ef3df94d428e1fbcbc1148de27e45c6a
Reviewed-on: https://chromium-review.googlesource.com/1107418
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568851}
[modify] https://crrev.com/56e6ce24e659c3ae732ecdfb36e25f9c02da5057/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.cc
[modify] https://crrev.com/56e6ce24e659c3ae732ecdfb36e25f9c02da5057/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.h
[modify] https://crrev.com/56e6ce24e659c3ae732ecdfb36e25f9c02da5057/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer_unittest.cc
[modify] https://crrev.com/56e6ce24e659c3ae732ecdfb36e25f9c02da5057/tools/metrics/histograms/histograms.xml