Fix XmppSignalStrategy to return correct result from SendStanza().

Fix XmppSignalStrategy to return correct result from SendStanza().

Previously XmppSignalStrategy::SendStanza() was returning true even
after write has failed on the socket. Failed socket write causes the
XMPP connection to be closed. When sending host offline reason that
results in HeartbeatSender being destroyed together with the IqSender
that was used to send the message, which was causing the crash in the
linked bug. Now IqSender::SendIq() can handle properly the case when
SignalStrategy::SendStanza() fails.

BUG=513951
Committed: https://crrev.com/f2382d91b38e9ea7469e3a9da289bb4e6e25cad8
Cr-Commit-Position: refs/heads/master@{#341243}

[Sergey Ulanov, 2015-07-28 23:53:18]

Patchset #1 (id:1) has been deleted

[Sergey Ulanov, 2015-07-28 23:54:08]

sergeyu@chromium.org changed reviewers:
+ dcaiafa@chromium.org

[dcaiafa, 2015-07-29 23:09:04]

lgtm

[Sergey Ulanov, 2015-07-30 01:36:50]

The CQ bit was checked by sergeyu@chromium.org

[commit-bot: I haz the power, 2015-07-30 01:37:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1258323003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1258323003/20001

[commit-bot: I haz the power, 2015-07-30 02:36:45]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-30 02:36:45]

Try jobs failed on following builders:
  linux_chromium_asan_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Sergey Ulanov, 2015-07-31 00:00:31]

The CQ bit was checked by sergeyu@chromium.org

[commit-bot: I haz the power, 2015-07-31 00:00:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1258323003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1258323003/20001

[commit-bot: I haz the power, 2015-07-31 00:35:38]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-07-31 00:36:19]

Patchset 1 (id:??) landed as
https://crrev.com/f2382d91b38e9ea7469e3a9da289bb4e6e25cad8
Cr-Commit-Position: refs/heads/master@{#341243}