Revert of Remove deprecated extension notification from content_settings

Revert of Remove deprecated extension notification from content_settings (patchset #6 id:240001 of https://codereview.chromium.org/276573011/)

Reason for revert:
Becuase this CL seems to break linux asan test
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_asan_rel/builds/882

Original issue's description:
> remove deprecated extension notification from content_settings.
>
> Use ExtensionRegistryObserver instead.
>
> R=bauerb@chromium.org
> BUG=354046, 354458
> TEST=unit_tests
>
> Committed: https://crrev.com/76ad58487578183de69a8caeb48b8a22dccad652
> Cr-Commit-Position: refs/heads/master@{#294768}

TBR=bauerb@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=354046, 354458
Committed: https://crrev.com/6026661e8bd401c953bdd4c70b2099f126251aa5
Cr-Commit-Position: refs/heads/master@{#294773}

[limasdf, 2014-09-15 00:21:10]

Created Revert of Remove deprecated extension notification from content_settings

[commit-bot: I haz the power, 2014-09-15 00:21:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/553263004/1

[commit-bot: I haz the power, 2014-09-15 00:21:18]

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

[commit-bot: I haz the power, 2014-09-15 00:21:18]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[limasdf, 2014-09-15 00:22:09]

limasdf@gmail.com changed reviewers:
+ tapted@chromium.org

[limasdf, 2014-09-15 00:23:05]

Trent, Could you take a look?

[jam, 2014-09-15 00:57:44]

jam@chromium.org changed reviewers:
+ jam@chromium.org

[jam, 2014-09-15 00:57:44]

this is also breaking all the debug windows bots. lgtm

[jam, 2014-09-15 00:57:51]

The CQ bit was checked by jam@chromium.org

[commit-bot: I haz the power, 2014-09-15 00:59:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/553263004/1

[commit-bot: I haz the power, 2014-09-15 01:04:00]

Committed patchset #1 (id:1) as af65d7ef4679c11609020e38b993e3c2db7abf06

[commit-bot: I haz the power, 2014-09-15 01:06:00]

Patchset 1 (id:??) landed as
https://crrev.com/6026661e8bd401c953bdd4c70b2099f126251aa5
Cr-Commit-Position: refs/heads/master@{#294773}

[tapted, 2014-09-15 01:19:14]

lgtm

[Bernhard Bauer, 2014-09-15 09:02:26]

Maaaaan, shutdown is hard. :-(

The good thing is, the ObserverList in ExtensionRegistry doesn't check whether
it's empty at destruction, so we could simply remove the unregister call.

Longer term, it might be good though to convert HostContentSettingsMap to a
KeyedService.

[limasdf, 2014-09-15 14:26:58]

Thank you guys who taking care of this. & sorry to bother you.

I'll take a look deeply again.