ServiceWorker: Issue a warning after initial execution of worker script

This patch enables the inspector to issue a warning when developers add a event handler after an initial evaluation of the ServiceWorker script.

BUG=401870
TEST=manually

Review URL: https://codereview.chromium.org/657213004

git-svn-id: svn://svn.chromium.org/blink/trunk@184227 bbb929c8-8fbe-4397-9dbb-9b2b20218538