[XHR] Reduce 'readystatechange' event firing.

Currently too many readystatechange events are fired and
it hurts performance.

BUG=336066

Review URL: https://codereview.chromium.org/428473005

git-svn-id: svn://svn.chromium.org/blink/trunk@179381 bbb929c8-8fbe-4397-9dbb-9b2b20218538