Rework EventSource CORS tests to be usable from Workers.

Improve EventSource-in-Workers test coverage by recasting existing
CORS tests as js-test.js tests, enabling them to also be testable from
within Workers. As was, we had no such Worker test coverage.

Also add two new tests covering failure-on-redirect handling: one due
to the CORS access check, the other due to CSP connect-src's.

R=abarth@chromium.org,mkwst@chromium.org
BUG=386520

Review URL: https://codereview.chromium.org/345813005

git-svn-id: svn://svn.chromium.org/blink/trunk@176950 bbb929c8-8fbe-4397-9dbb-9b2b20218538