Add a histogram to measure number of synthetic UMA trials.

Add a histogram to measure number of synthetic UMA trials.

Also refactors some common code into a method in MetricsService.

BUG=400357
Committed: https://crrev.com/88aa9330b9dcc79911d607c5d7156d5345d69633
Cr-Commit-Position: refs/heads/master@{#297302}

[Alexei Svitkine (slow), 2014-09-29 15:38:37]

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

[Alexei Svitkine (slow), 2014-09-29 20:15:23]

Patchset #2 (id:20001) has been deleted

[jwd, 2014-09-29 21:07:38]

lgtm

[Alexei Svitkine (slow), 2014-09-29 21:12:24]

The CQ bit was checked by asvitkine@chromium.org

[commit-bot: I haz the power, 2014-09-29 21:13:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/612883003/1

[commit-bot: I haz the power, 2014-09-29 23:29:25]

Committed patchset #1 (id:1) as f859252fec4af2ef22055b046f81865418e32e7c

[commit-bot: I haz the power, 2014-09-29 23:35:17]

Patchset 1 (id:??) landed as
https://crrev.com/88aa9330b9dcc79911d607c5d7156d5345d69633
Cr-Commit-Position: refs/heads/master@{#297302}