[GOM] Warn when the roll is running behind

This change introduces a UI that appears when the last roll
is getting old. The UI is not shown by default to avoid
visual clutter and its visual attribute matches the urgency
(currently 2 levels: warning / urgent).

Review URL: https://codereview.chromium.org/316033002

git-svn-id: svn://svn.chromium.org/blink/trunk@175634 bbb929c8-8fbe-4397-9dbb-9b2b20218538