Reland "Handle unsucessful JsonPrefStore initialization in SupervisedUserSettingsService."

Reland "Handle unsucessful JsonPrefStore initialization in SupervisedUserSettingsService."

> According to crash reports, it's not very common but still possible for the JsonPrefStore used by SupervisedUserSettingsService to fail initialization (which happens if the profile directory doesn't exist).

Review URL: https://codereview.chromium.org/960233003

This reverts commit 3490af836c15f44229dedea4b3d80c1869d6e1fe.

TBR=treib@chromium.org
BUG=463646
Committed: https://crrev.com/4f3a803c5fcdd82dfb7e6a134106ddde1dd7525c
Cr-Commit-Position: refs/heads/master@{#322105}

[Bernhard Bauer, 2015-03-24 22:02:29]

bauerb@chromium.org changed reviewers:
+ treib@chromium.org

[Bernhard Bauer, 2015-03-24 22:03:08]

Marc: TBR (reland with no changes to the original version)

[Bernhard Bauer, 2015-03-24 22:03:15]

The CQ bit was checked by bauerb@chromium.org

[commit-bot: I haz the power, 2015-03-24 22:03:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1016423003/1

[commit-bot: I haz the power, 2015-03-25 00:44:58]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-25 00:46:01]

Patchset 1 (id:??) landed as
https://crrev.com/4f3a803c5fcdd82dfb7e6a134106ddde1dd7525c
Cr-Commit-Position: refs/heads/master@{#322105}