Add resource_coordinator::NowTicks().

This CL introduces resource_coordinator::NowTicks(), which is now
the only source of time in chrome/browser/resource_coordinator/.
This design is simpler than passing a TickClock* object across
objects and will facilitate the TabManager refactor.

Bug: 775644 
Change-Id: I8759879e95488cce3a0d591b075b02fa6f18eea9
Reviewed-on: https://chromium-review.googlesource.com/723674
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514261}
12 files changed