Fixes and reenables SystemUse24HourClockPolicyTest

Fixes and reenables SystemUse24HourClockPolicyTest

Adds code which attempts to exit by closing all browser instances and gracefully shuts down the test.

BUG=450651
Committed: https://crrev.com/a55d03762d344abc6742c82aba04cd7b2c48224c
Cr-Commit-Position: refs/heads/master@{#313056}

[cschuet (SLOW), 2015-01-26 09:09:05]

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

[Andrew T Wilson (Slow), 2015-01-26 09:19:27]

LGTM.

Ideally we'd have some kind of LoginScreenBrowserTest base class that does this
- if you are feeling extra motivated and want to define something like this and
change all the existing tests that are manually calling AttemptExit() to instead
derive from this class, I'd be happy to review that in a separate CL.

Or don't worry about it, it's not like it's the biggest issue in the codebase.

[cschuet (SLOW), 2015-01-26 09:27:20]

The CQ bit was checked by cschuet@chromium.org

[commit-bot: I haz the power, 2015-01-26 09:28:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/874933002/1

[commit-bot: I haz the power, 2015-01-26 10:13:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-26 10:14:22]

Patchset 1 (id:??) landed as
https://crrev.com/a55d03762d344abc6742c82aba04cd7b2c48224c
Cr-Commit-Position: refs/heads/master@{#313056}