Reduce number of roundtrips required in XMPP handshake

Reduce number of roundtrips required in XMPP handshake

Previously XMPP handshake implemented in XmppLoginHandler required
7 roundtrips (excluding TLS handshake). With this change it will be
sending each message as soon as possible without waiting response to
the previous message. This effectivelly reduces number of required
roundtrips to 3.

BUG=274652
Committed: https://crrev.com/a55e1da00db331b8199a6a396fae7fc7a0ad715c
Cr-Commit-Position: refs/heads/master@{#293389}

[Sergey Ulanov, 2014-09-03 00:53:33]

sergeyu@chromium.org changed reviewers:
+ jamiewalch@chromium.org, kelvinp@chromium.org

[kelvinp, 2014-09-03 01:10:00]

lgtm

[Sergey Ulanov, 2014-09-04 21:08:21]

The CQ bit was checked by sergeyu@chromium.org

[commit-bot: I haz the power, 2014-09-04 21:10:22]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sergeyu@chromium.org/534853002/20001

[commit-bot: I haz the power, 2014-09-05 01:18:42]

Committed patchset #2 (id:20001) as 6ee74d0885a5b7c68e696addac05d09b4b71d8cf

[commit-bot: I haz the power, 2014-09-10 03:34:38]

Patchset 2 (id:??) landed as
https://crrev.com/a55e1da00db331b8199a6a396fae7fc7a0ad715c
Cr-Commit-Position: refs/heads/master@{#293389}