Fix FakeSigninManager::SignOut

Fix FakeSigninManager::SignOut

FakeSigninManager::SignOut was never updated to clear the account id on
SignOut. This result in calls to FakeSigninManager::IsAuthenticated
returning true after even after a SignOut.

BUG=

Committed: https://crrev.com/090c49d7c2001d5279218ecdc6c046cb78cf7c51
Cr-Commit-Position: refs/heads/master@{#319077}

[bzanotti, 2015-03-04 14:36:15]

bzanotti@chromium.org changed reviewers:
+ atwilson@chromium.org

[bzanotti, 2015-03-04 14:36:16]

Please take a look.

[Andrew T Wilson (Slow), 2015-03-04 17:03:28]

lgtm

[bzanotti, 2015-03-04 17:11:43]

The CQ bit was checked by bzanotti@chromium.org

[commit-bot: I haz the power, 2015-03-04 17:12:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/980583003/1

[commit-bot: I haz the power, 2015-03-04 17:15:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 17:16:23]

Patchset 1 (id:??) landed as
https://crrev.com/090c49d7c2001d5279218ecdc6c046cb78cf7c51
Cr-Commit-Position: refs/heads/master@{#319077}