[XHR] Call internalAbort() before firing events

If we fire internalAbort() after handleRequestError(), a new request
initiated by the event handler reusing the same XHR instance will be
aborted.

BUG=402855

Review URL: https://codereview.chromium.org/462053002

git-svn-id: svn://svn.chromium.org/blink/trunk@180326 bbb929c8-8fbe-4397-9dbb-9b2b20218538