Add a new UMA to GCM

Add a new UMA to GCM

This is used to find out the GCM usage from non-signed-in users.

BUG=384041
TEST=none

Committed: https://crrev.com/9a5d1544434f81316604dc8b35fb6380d0d5a773
Cr-Commit-Position: refs/heads/master@{#297045}

[jianli, 2014-09-26 19:00:49]

jianli@chromium.org changed reviewers:
+ asvitkine@chromium.org, zea@chromium.org

[jianli, 2014-09-26 19:00:49]

asvitkine for new UMA
zea for GCM hookup

[Alexei Svitkine (slow), 2014-09-26 19:59:52]

lgtm

[Nicolas Zea, 2014-09-26 20:30:13]

LGTM with some nits

https://codereview.chromium.org/605363002/diff/1/tools/metrics/histograms/his...
File tools/metrics/histograms/histograms.xml (right):

https://codereview.chromium.org/605363002/diff/1/tools/metrics/histograms/his...
tools/metrics/histograms/histograms.xml:9231: +    Indicates whether the user
signs in or not when GCM is used.
nit: "signs in" -> was signed in

Also "is used" -> "started up".

[jianli, 2014-09-26 20:36:28]

The CQ bit was checked by jianli@chromium.org

[commit-bot: I haz the power, 2014-09-26 20:38:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/605363002/20001

[commit-bot: I haz the power, 2014-09-26 21:50:36]

Committed patchset #2 (id:20001) as 83f433b84acbd2cb53a123672e9a1c754c366a7f

[commit-bot: I haz the power, 2014-09-26 21:51:19]

Patchset 2 (id:??) landed as
https://crrev.com/9a5d1544434f81316604dc8b35fb6380d0d5a773
Cr-Commit-Position: refs/heads/master@{#297045}