Fix network portal detector crash on shutdown

Fix network portal detector crash on shutdown

Note: This doesn't occur under normal circumstances, but if a DBus crash
(or something else) causes an immediate shutdown, we sometimes crash in
NetworkPortalDetector::Get() which obscures the actual problem.

This also updates the portal detector logging to use device_event_log.h.

BUG=none

Committed: https://crrev.com/dacf4ec47dcb83a6eae892bb2f1b104d62d05782
Cr-Commit-Position: refs/heads/master@{#312900}

[stevenjb, 2015-01-22 18:25:03]

stevenjb@chromium.org changed reviewers:
+ ygorshenin@chromium.org

[ygorshenin1, 2015-01-23 15:07:37]

lgtm

[stevenjb, 2015-01-23 18:13:25]

The CQ bit was checked by stevenjb@chromium.org

[commit-bot: I haz the power, 2015-01-23 18:15:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/865813005/1

[commit-bot: I haz the power, 2015-01-23 19:43:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-23 19:46:27]

Patchset 1 (id:??) landed as
https://crrev.com/dacf4ec47dcb83a6eae892bb2f1b104d62d05782
Cr-Commit-Position: refs/heads/master@{#312900}