Deprecate support for xhr.withCredentials for synchronous requests

As per XML Level2 spec, withCredentials flag cannot be set on sync request.
http://xhr.spec.whatwg.org/#the-withcredentials-attribute
However for time being add useCounter and throw console warning to web developers.

Intent to deprecate - https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/BOgLhbdljU4

BUG=383483

Review URL: https://codereview.chromium.org/329323002

git-svn-id: svn://svn.chromium.org/blink/trunk@176532 bbb929c8-8fbe-4397-9dbb-9b2b20218538