Add a test for the timing of start events with varying delays

I couldn't find a test which checks this explicitly.

BUG=248938
R=timloh@chromium.org

Review URL: https://codereview.chromium.org/52053002

git-svn-id: svn://svn.chromium.org/blink/trunk@160995 bbb929c8-8fbe-4397-9dbb-9b2b20218538