Fix tsan deadlock in base_unittest

base/metrics/field_trial_param_associator.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/metrics/field_trial_param_associator.h"
 
 #include "base/metrics/field_trial.h"
 
 namespace base {
 
@@ skipping 48 matching lines @@
 
   const FieldTrialKey key(trial_name, group_name);
   if (!ContainsKey(field_trial_params_, key))
     return false;
 
   *params = field_trial_params_[key];
   return true;
 }
 
 void FieldTrialParamAssociator::ClearAllParamsForTesting() {
-  AutoLock scoped_lock(lock_);
-  field_trial_params_.clear();
+  {
+    AutoLock scoped_lock(lock_);
+    field_trial_params_.clear();
+  }
   FieldTrialList::ClearParamsFromSharedMemoryForTesting();
 }
 
 void FieldTrialParamAssociator::ClearAllCachedParamsForTesting() {
   AutoLock scoped_lock(lock_);
   field_trial_params_.clear();
 }
 
 }  // namespace base