Stop throwing an exception when getting XMLHttpRequest.status and statusText.

Latest XHR spec says that XHR should return 0 and an empty string when
it's in UNSENT or OPENED state or error flag is set.

BUG=223690

Review URL: https://codereview.chromium.org/23750004

git-svn-id: svn://svn.chromium.org/blink/trunk@158657 bbb929c8-8fbe-4397-9dbb-9b2b20218538