Factor out delivery of throttled progress event.

To prepare for follow-on changes to progress event handling, split out
the delivery of single XHR progress events into a separate method.

R=
BUG=315488

Review URL: https://codereview.chromium.org/75103006

git-svn-id: svn://svn.chromium.org/blink/trunk@162253 bbb929c8-8fbe-4397-9dbb-9b2b20218538