Popup metrics: track visible time for all tabs

This will help as a reference point for other visibility metrics at the
tab layer (namely popups and tab-unders).

This CL contains a minor refactor, so that the ScopedVisibilityTrackers
no longer own their tick clocks.

Bug: 661629
Change-Id: I879fe751661876442b609bad9008d16af52ba2f1
Reviewed-on: https://chromium-review.googlesource.com/694741
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505886}
10 files changed