[Smart Lock] Add a description for the promo notification histogram.

[Smart Lock] Add a description for the promo notification histogram.

BUG=450508
TEST=none
R=tengs@chromium.org

Committed: https://crrev.com/800bd1ef4666b67c62f864bb3e6b67b05f1cbc1c
Cr-Commit-Position: refs/heads/master@{#317981}

[Tim Song, 2015-02-23 19:33:30]

lgtm

[Ilya Sherman, 2015-02-23 19:34:30]

The CQ bit was checked by isherman@chromium.org

[commit-bot: I haz the power, 2015-02-23 19:35:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/938223002/1

[commit-bot: I haz the power, 2015-02-23 19:39:03]

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

[commit-bot: I haz the power, 2015-02-23 19:39:04]

Try jobs failed on following builders:
  ios_dbg_simulator on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[Ilya Sherman, 2015-02-25 01:54:29]

The CQ bit was checked by isherman@chromium.org

[Ilya Sherman, 2015-02-25 01:54:29]

The patchset sent to the CQ was uploaded after l-g-t-m from tengs@chromium.org
Link to the patchset: https://codereview.chromium.org/938223002/#ps20001 (title:
"Rebase")

[commit-bot: I haz the power, 2015-02-25 01:55:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/938223002/20001

[commit-bot: I haz the power, 2015-02-25 05:03:08]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-25 05:04:33]

Patchset 2 (id:??) landed as
https://crrev.com/800bd1ef4666b67c62f864bb3e6b67b05f1cbc1c
Cr-Commit-Position: refs/heads/master@{#317981}