Fix tsan deadlock in base_unittest

BUG=672859

Review-Url: https://codereview.chromium.org/2568533002
Cr-Commit-Position: refs/heads/master@{#437623}

base/metrics/field_trial_param_associator.cc

@@ -65,8 +65,10 @@ bool FieldTrialParamAssociator::GetFieldTrialParamsWithoutFallback(
 }
 
 void FieldTrialParamAssociator::ClearAllParamsForTesting() {
-  AutoLock scoped_lock(lock_);
-  field_trial_params_.clear();
+  {
+    AutoLock scoped_lock(lock_);
+    field_trial_params_.clear();
+  }
   FieldTrialList::ClearParamsFromSharedMemoryForTesting();
 }