Add a histogram to count requests to variations server.

Add a histogram to count requests to variations server.

This will let us know what percentage of requests are a
result of periodic polling vs. the initial requests when
Chrome starts up.

BUG=483057
Committed: https://crrev.com/83104bf0bf4c6ced3f8099da4e3f01835ec35237
Cr-Commit-Position: refs/heads/master@{#329909}

[Alexei Svitkine (slow), 2015-05-14 16:50:53]

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

[Steven Holte, 2015-05-14 18:40:34]

lgtm

[Alexei Svitkine (slow), 2015-05-14 18:53:10]

The CQ bit was checked by asvitkine@chromium.org

[commit-bot: I haz the power, 2015-05-14 18:54:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1139133002/1

[commit-bot: I haz the power, 2015-05-14 19:55:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-14 19:56:07]

Patchset 1 (id:??) landed as
https://crrev.com/83104bf0bf4c6ced3f8099da4e3f01835ec35237
Cr-Commit-Position: refs/heads/master@{#329909}