[XHR] readyState must change to LOADING before "progress" event is dispatched.

As specified in http://xhr.spec.whatwg.org/#the-send()-method , changing state
must precede dispatching a "progress" progress event.

BUG=409544
R=tyoshino

Review URL: https://codereview.chromium.org/532463002

git-svn-id: svn://svn.chromium.org/blink/trunk@181217 bbb929c8-8fbe-4397-9dbb-9b2b20218538