Add UMA stats for icon-based NTP.

Add UMA stats for icon-based NTP.

These stats are currently used by internal code, but that code
will be moved into Chromium soon.

BUG=483656
Committed: https://crrev.com/01d9b43f17709a27a936474a3808f7bcdab6db35
Cr-Commit-Position: refs/heads/master@{#329701}

[newt (away), 2015-05-13 00:45:47]

newt@chromium.org changed reviewers:
+ asvitkine@chromium.org, beaudoin@chromium.org

[newt (away), 2015-05-13 00:45:47]

PTAL

[beaudoin, 2015-05-13 01:48:03]

LGTM

[Alexei Svitkine (slow), 2015-05-13 14:37:45]

Can you link to the CL that implemented this? Usually it's best to have the
histograms.xml change in the same CL that implements the metrics, unless they're
being implemented in another repository? CL description doesn't give much
context.

[newt (away), 2015-05-13 17:47:07]

The stats are currently used here:
https://chrome-internal-review.googlesource.com/#/c/216165/  but that code will
be moved into Chromium soon. Plus, these stats will likely be used by other
platforms, so I think it makes sense to add them here. I updated the commit
message to explain a bit.

[Alexei Svitkine (slow), 2015-05-13 17:50:35]

lgtm

[newt (away), 2015-05-13 17:52:24]

The CQ bit was checked by newt@chromium.org

[commit-bot: I haz the power, 2015-05-13 17:54:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1123243006/1

[commit-bot: I haz the power, 2015-05-13 19:57:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-13 19:58:11]

Patchset 1 (id:??) landed as
https://crrev.com/01d9b43f17709a27a936474a3808f7bcdab6db35
Cr-Commit-Position: refs/heads/master@{#329701}