Fix the documentation of ExtensionRegistryObserver::OnExtensionUnloaded.

Fix the documentation of ExtensionRegistryObserver::OnExtensionUnloaded.

The documentation didn't match the implementation, e.g. of
ExtensionService::TrackTerminatedExtension .

Committed: https://crrev.com/ca65e0ad7834d79d04f8d8d0e846c040a1846145
Cr-Commit-Position: refs/heads/master@{#322030}

[pneubeck (no reviews), 2015-03-24 09:19:09]

Patchset #1 (id:1) has been deleted

[pneubeck (no reviews), 2015-03-24 09:35:10]

Patchset #1 (id:20001) has been deleted

[pneubeck (no reviews), 2015-03-24 09:35:34]

pneubeck@chromium.org changed reviewers:
+ rockot@chromium.org

[pneubeck (no reviews), 2015-03-24 09:35:35]

ptal

[Ken Rockot(use gerrit already), 2015-03-24 16:51:21]

lgtm

[pneubeck (no reviews), 2015-03-24 17:01:25]

The CQ bit was checked by pneubeck@chromium.org

[commit-bot: I haz the power, 2015-03-24 17:02:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1026223003/40001

[commit-bot: I haz the power, 2015-03-24 18:15:43]

Committed patchset #1 (id:40001)

[commit-bot: I haz the power, 2015-03-24 18:16:22]

Patchset 1 (id:??) landed as
https://crrev.com/ca65e0ad7834d79d04f8d8d0e846c040a1846145
Cr-Commit-Position: refs/heads/master@{#322030}