Add an UMA stat to be able to see if the User pods are show on start screen,

Add an UMA stat to be able to see if the User pods are show on start screen,
and if not, what disabled them: the local setting or domain policy.

BUG=470383
Committed: https://crrev.com/46bbbcd70fbf85d2624ad0a4b678255efa9fe5e9
Cr-Commit-Position: refs/heads/master@{#324206}

[Ivan Podogov, 2015-04-07 13:05:35]

ginkage@chromium.org changed reviewers:
+ dpolukhin@chromium.org, isherman@chromium.org

[Ivan Podogov, 2015-04-07 14:18:46]

PTAL

[Dmitry Polukhin, 2015-04-07 14:20:07]

lgtm

[Ilya Sherman, 2015-04-07 21:31:18]

lgtm

https://codereview.chromium.org/1064843003/diff/1/chrome/browser/chromeos/log...
File chrome/browser/chromeos/login/session/user_session_manager.cc (right):

https://codereview.chromium.org/1064843003/diff/1/chrome/browser/chromeos/log...
chrome/browser/chromeos/login/session/user_session_manager.cc:13: #include
"base/metrics/histogram.h"
nit: Please include histogram_macros instead

[Ivan Podogov, 2015-04-08 08:04:43]

The CQ bit was checked by ginkage@chromium.org

[Ivan Podogov, 2015-04-08 08:04:44]

The patchset sent to the CQ was uploaded after l-g-t-m from
dpolukhin@chromium.org, isherman@chromium.org
Link to the patchset: https://codereview.chromium.org/1064843003/#ps20001
(title: "Nit.")

[commit-bot: I haz the power, 2015-04-08 08:05:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1064843003/20001

[commit-bot: I haz the power, 2015-04-08 10:02:34]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-08 10:03:26]

Patchset 2 (id:??) landed as
https://crrev.com/46bbbcd70fbf85d2624ad0a4b678255efa9fe5e9
Cr-Commit-Position: refs/heads/master@{#324206}