Collapsible offline content list on dino page

Allows the offline content list on the dino page to be collapsed by the
user, mirroring the UX used on the NTP for collapsing the list of
suggested articles.

These two collapsible lists also share the same PrefsService setting so
that the expanded/collapsed state of one is reflected on the other.

This also fixes a small issue with the background color appearing
between a suggestion image and its border.

Bug: 852872
Change-Id: I47ed7abaa777fe233ec79153a033482812ae478a
Reviewed-on: https://chromium-review.googlesource.com/c/1318630
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Cait Phillips <caitkp@chromium.org>
Reviewed-by: Dan H <harringtond@google.com>
Reviewed-by: Edward Jung <edwardjung@chromium.org>
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609837}