GitLab

This implements a periodic background task that attempts to fetch the
Explore Sites catalog approximately daily.  Fetch implementation is left
to a future CL, as is initiating the background task.

Bug: 867488
Change-Id: I808a7bacd9f20c3c1dbcaf84acd413eb7353c644
Reviewed-on: https://chromium-review.googlesource.com/1236506Reviewed-by: Filip Gorski <fgorski@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593071}