Rework CORS <link rel=stylesheet> tests as js-tests.

Attempt to get at the flakiness exhibited by one or more of these tests,
on Windows mainly.

R=tyoshino@chromium.org
BUG=394208

Review URL: https://codereview.chromium.org/399653002

git-svn-id: svn://svn.chromium.org/blink/trunk@178341 bbb929c8-8fbe-4397-9dbb-9b2b20218538