Call CheckTrialGroup only under lock.

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index a0cfdce70cf6aee312f2392e3643be0e16eb8199..78862fa683520bc5d96764e093d7fce54386e12a 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -615,8 +615,11 @@ void FieldTrialList::NotifyFieldTrialGroupSelection(FieldTrial* field_trial) {
   if (!field_trial->enable_field_trial_)
     return;
 
-  CheckTrialGroup(field_trial->trial_name(), field_trial->group_name_internal(),
-                  &global_->seen_states_);
+  {
+    AutoLock auto_lock(global_->lock_);
+    CheckTrialGroup(field_trial->trial_name(),
+                    field_trial->group_name_internal(), &global_->seen_states_);
+  }
   global_->observer_list_->Notify(
       FROM_HERE, &FieldTrialList::Observer::OnFieldTrialGroupFinalized,
       field_trial->trial_name(), field_trial->group_name_internal());