Fix crash with forcing variations when the trial exists.

Fix crash with forcing variations when the trial exists.

This can happen, for example, when a user used the
--force-fieldtrials= command-line to set a different
group than what they have selected from chrome://flags.

Includes a test.

BUG=409976
Committed: https://crrev.com/2955e423a8d8c0c25af89585ff21394fc77ccb9e
Cr-Commit-Position: refs/heads/master@{#293173}

[Alexei Svitkine (slow), 2014-09-02 21:42:01]

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

[jwd, 2014-09-03 14:31:58]

lgtm

[Alexei Svitkine (slow), 2014-09-03 14:35:51]

The CQ bit was checked by asvitkine@chromium.org

[commit-bot: I haz the power, 2014-09-03 14:36:40]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asvitkine@chromium.org/529183002/1

[commit-bot: I haz the power, 2014-09-03 18:16:50]

Committed patchset #1 (id:1) as 2c5978f930fe9159ab8df1588fcfa8f37fef5e98

[commit-bot: I haz the power, 2014-09-10 03:26:57]

Patchset 1 (id:??) landed as
https://crrev.com/2955e423a8d8c0c25af89585ff21394fc77ccb9e
Cr-Commit-Position: refs/heads/master@{#293173}