Revert of [XHR] Reduce 'readystatechange' event firing. (patchset #9 of https://codereview.chromium.org/428473005/)

Reason for revert:
This change breaks legacy applications.

Original issue's description:
> [XHR] Reduce 'readystatechange' event firing.
> 
> Currently too many readystatechange events are fired and
> it hurts performance.
> 
> BUG=336066
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179381

Review URL: https://codereview.chromium.org/486133002

git-svn-id: svn://svn.chromium.org/blink/trunk@180554 bbb929c8-8fbe-4397-9dbb-9b2b20218538