Add 'Managed by' UI on chrome://bookmarks

Adds a message for managed users at the bottom of the sidebar on the
chrome://bookmarks page.

Enterprise users will now see a message that says "Your browser is
managed by your organization", with a link they can click to view a help
center article.

The new UI is hidden behind a flag so that most users don't see this WIP
feature just yet.

Bug: 893628
Change-Id: Ic53e0eef22254c6de4aca7067f7ef01d5062870d
Reviewed-on: https://chromium-review.googlesource.com/c/1378638
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617497}