sheriff-o-matic: add a help link
Adds a link in the toolbar to http://www.chromium.org/developers/tree-sheriffs/sheriff-details-chromium

BUG=420469

Review URL: https://codereview.chromium.org/696813002

git-svn-id: svn://svn.chromium.org/blink/trunk@184811 bbb929c8-8fbe-4397-9dbb-9b2b20218538