Add last update time to Sheriff-O-Matic.

This uses the "date" given in the http://auto-sheriff.appspot.com/data page to
show how recent the failure information is.

I intend to add some styling when the data is more than 5 minutes old, but I'm
pretty new to Polymer and want to make sure this is on track first.

BUG=399712
NOTRY=true

Review URL: https://codereview.chromium.org/453543002

git-svn-id: svn://svn.chromium.org/blink/trunk@179953 bbb929c8-8fbe-4397-9dbb-9b2b20218538