XHR: separate delayed progress event flushing from dispatching of new.

XHR: separate delayed progress event flushing from dispatching of new.

The throttled delivery of progress events may defer progress event
delivery so as to pace their dispatching down to the frequency that
the spec suggests. When dispatching a later 'readystatechange' event,
if no deferred progress event was present, a new one would be created
and dispatched. Achieving the same event sequence despite the
lazy/deferred delivery of 'progress' events.

There are two steps involved here: the potential expedition of the
deferred progress event and the creation & delivery of a new one.
Tease these two notions apart here by allowing only flushing of
a deferred progress event to happen.

This weaker form of flushing is needed by follow-on changes, which
alters how these 'progress' progress events are dispatched wrt the
'readystatechange' XHR event when handling request errors and aborts.

R=
BUG=315488
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=162270

[sof, 2013-11-18 22:27:50]

At your leisure, please take a look.

(Split out from crrev.com/61793010)

[abarth-chromium, 2013-11-18 23:14:27]

lgtm

[commit-bot: I haz the power, 2013-11-18 23:14:32]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sigbjornf@opera.com/62193004/1

[commit-bot: I haz the power, 2013-11-19 06:34:49]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sigbjornf@opera.com/62193004/1

[commit-bot: I haz the power, 2013-11-19 08:02:39]

Change committed as 162270