Don't run sheriff-o-matic unittests until polymer-ready.

Polymer isn't ready to do anything until this event fires,
which is not necessarily after the window's load event.

NOTRY=true

Review URL: https://codereview.chromium.org/396783007

git-svn-id: svn://svn.chromium.org/blink/trunk@178243 bbb929c8-8fbe-4397-9dbb-9b2b20218538