Disable HTTP authentication dialog for XHR with credentials

According to the XHR[1] and Fetch[2] specifications, a request made with
URL credentials should not trigger the browser's authentication dialog
upon authentication failure, but rather return the 401 response.

[1] http://xhr.spec.whatwg.org/
[2] http://fetch.spec.whatwg.org/

BUG=31582

Review URL: https://codereview.chromium.org/143903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247117 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247117