Fix crash on Enrollment screen.

Fix crash on Enrollment screen.

This is a sequel to crbug/470977, with a similar cause. This time, crash
is induced by WizardController re-creation, so the old NetworkErrorModel
pointers become invalid. Lesson learned: WizardController should NOT be
the owner of an ErrorScreen instance.

BUG=479666
Committed: https://crrev.com/97422a9583d83ea837f889a2e23adb157092f9ae
Cr-Commit-Position: refs/heads/master@{#326317}

[Ivan Podogov, 2015-04-22 14:42:50]

ginkage@chromium.org changed reviewers:
+ nkostylev@chromium.org

[Ivan Podogov, 2015-04-22 14:42:57]

PTAL

[Nikita (slow), 2015-04-22 14:48:43]

The CQ bit was checked by nkostylev@chromium.org

[Nikita (slow), 2015-04-22 14:48:44]

lgtm

[commit-bot: I haz the power, 2015-04-22 14:49:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1098303002/1

[commit-bot: I haz the power, 2015-04-22 15:37:51]

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

[commit-bot: I haz the power, 2015-04-22 15:37:53]

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

[Ivan Podogov, 2015-04-22 15:52:27]

The CQ bit was checked by ginkage@chromium.org

[Ivan Podogov, 2015-04-22 15:52:28]

The patchset sent to the CQ was uploaded after l-g-t-m from
nkostylev@chromium.org
Link to the patchset: https://codereview.chromium.org/1098303002/#ps20001
(title: "Fix test compilation.")

[commit-bot: I haz the power, 2015-04-22 15:52:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1098303002/20001

[commit-bot: I haz the power, 2015-04-22 16:55:53]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-22 16:56:51]

Patchset 2 (id:??) landed as
https://crrev.com/97422a9583d83ea837f889a2e23adb157092f9ae
Cr-Commit-Position: refs/heads/master@{#326317}