Verify that video.crossOrigin performs no preflighting.

Verify that a video element CORS fetch doesn't generate a preflight
request and loads the cross-origin resource ok.

R=jochen,kbr
BUG=173727

Review URL: https://codereview.chromium.org/75153002

git-svn-id: svn://svn.chromium.org/blink/trunk@162293 bbb929c8-8fbe-4397-9dbb-9b2b20218538