Drop deprecation message for xhr.withCredentials for sync requests

The spec was changed due to a compat problem in Gecko, and Gecko has
already changed as well, "since no one except for us bothered to
implement the spec":
https://bugzilla.mozilla.org/show_bug.cgi?id=1260515
https://github.com/whatwg/xhr/issues/66

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

BUG=383483

Review-Url: https://codereview.chromium.org/2025903002
Cr-Commit-Position: refs/heads/master@{#397085}
14 files changed