[XHR] readystatechange events' interface must be Event, not ProgressEvent.

Event interface must be used for 'readystatechange' events in XMLHttpRequest.
This behavior originates at [1] but the behavior change looks a mistake.

1: https://chromium.googlesource.com/chromium/blink/+/bb39df6f38f867994d2c87e6279fc5d2f566d71f%5E%21

BUG=411817
R=sof, eseidel, tyoshino

Review URL: https://codereview.chromium.org/561113002

git-svn-id: svn://svn.chromium.org/blink/trunk@182384 bbb929c8-8fbe-4397-9dbb-9b2b20218538