GitLab

This CL slightly updates PageEndReason3 (new version from 2):
* HIDDEN is removed as a page end reason due to the fact that we no
longer flush the metric when the page is hidden.
* APP_ENTER_BACKGROUND is added as a reason as the metric can be
reported when this happens, and it is not a page closure per se.

UKM collection review:
https://docs.google.com/document/d/1nGbQlG5bdrKn_vFzZxIIiIzAGbwooC1n2S4DHYmVivI/

Bug: 1049260
Change-Id: I3ff183d73f36287f3a99590abd552ea15d37e252
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2630746Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844317}