Prevents sending of 'orgin' in the "Access-Control-Request-Headers" when preflight requests are made.

As per the W3C CORS Spec(http://www.w3.org/TR/cors/#cross-origin-request-with-preflight-0), 
the "Access-Control-Request-Headers" should contain only author request headers. Origin header was
set(by UA) for the actual request before the preflight request was done. 

Avoid setting of Origin header for the actual request in case of preflight. Origin header is anyways
set for the actual request in preflightSuccess(). This makes the behavior same as Mozilla Firefox browser.

R=abarth@chromium.org, kbr@chromium.org, bbudge@chromium.org
BUG=258828

Review URL: https://chromiumcodereview.appspot.com/18595008

git-svn-id: svn://svn.chromium.org/blink/trunk@154346 bbb929c8-8fbe-4397-9dbb-9b2b20218538