Rework EventSource tests for better Worker test coverage.

Follow on from https://codereview.chromium.org/340363003/ and recast
the remaining Document-agnostic EventSource tests to also be usable from
a Worker context.

R=
BUG=386520

Review URL: https://codereview.chromium.org/347043002

git-svn-id: svn://svn.chromium.org/blink/trunk@176665 bbb929c8-8fbe-4397-9dbb-9b2b20218538