chromeos: Document Uptime.DBusCrash histogram.

chromeos: Document Uptime.DBusCrash histogram.

Document a new histogram that will be reported from a Chrome
OS Upstart script when the system is automatically rebooted
after a dbus-daemon crash.

BUG=432980
Committed: https://crrev.com/fadb365423695fb4242524aa2aafe409026ec42b
Cr-Commit-Position: refs/heads/master@{#310769}

[Daniel Erat, 2015-01-07 22:44:21]

derat@chromium.org changed reviewers:
+ asvitkine@chromium.org

[Daniel Erat, 2015-01-08 01:30:00]

(updated this to be a histogram instead)

[Alexei Svitkine (slow), 2015-01-08 15:18:59]

lgtm

[Daniel Erat, 2015-01-09 13:56:29]

The CQ bit was checked by derat@chromium.org

[commit-bot: I haz the power, 2015-01-09 13:57:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/807933005/20001

[commit-bot: I haz the power, 2015-01-09 14:56:40]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-09 14:57:26]

Patchset 2 (id:??) landed as
https://crrev.com/fadb365423695fb4242524aa2aafe409026ec42b
Cr-Commit-Position: refs/heads/master@{#310769}