Call CheckTrialGroup only under lock.

Call CheckTrialGroup only under lock.

Since this code can be called from multiple
threads, we should hold a lock when calling
CheckTrialGroup since it modifies a std::map.

The other call site was already holding the
lock.

BUG=595264
Review URL: https://codereview.chromium.org/1810593002

Cr-Commit-Position: refs/heads/master@{#381535}
(cherry picked from commit cba00314e57ab31bc5a7f766e9bb8d5760d106ac)

Committed: https://chromium.googlesource.com/chromium/src/+/874f686fc9a9a937e3779757dbfeb50e69440582

[Alexei Svitkine (slow), 2016-04-15 21:38:00]

Description was changed from

==========
Call CheckTrialGroup only under lock.

Since this code can be called from multiple
threads, we should hold a lock when calling
CheckTrialGroup since it modifies a std::map.

The other call site was already holding the
lock.

BUG=595264

Review URL: https://codereview.chromium.org/1810593002

Cr-Commit-Position: refs/heads/master@{#381535}
(cherry picked from commit cba00314e57ab31bc5a7f766e9bb8d5760d106ac)
==========

to

==========
Call CheckTrialGroup only under lock.

Since this code can be called from multiple
threads, we should hold a lock when calling
CheckTrialGroup since it modifies a std::map.

The other call site was already holding the
lock.

BUG=595264

Review URL: https://codereview.chromium.org/1810593002

Cr-Commit-Position: refs/heads/master@{#381535}
(cherry picked from commit cba00314e57ab31bc5a7f766e9bb8d5760d106ac)

Committed:
https://chromium.googlesource.com/chromium/src/+/874f686fc9a9a937e3779757dbfe...
==========

[Alexei Svitkine (slow), 2016-04-15 21:38:02]

Committed patchset #1 (id:1) manually as
874f686fc9a9a937e3779757dbfeb50e69440582.