Fix tsan deadlock in base_unittest

base/metrics/field_trial_param_associator.cc

Index: base/metrics/field_trial_param_associator.cc
diff --git a/base/metrics/field_trial_param_associator.cc b/base/metrics/field_trial_param_associator.cc
index 0230b8a8a8d39082d166052eb00303e4ff03919e..7e3a15febb28d2a42797c9bc2a67d39f67726892 100644
--- a/base/metrics/field_trial_param_associator.cc
+++ b/base/metrics/field_trial_param_associator.cc
@@ -66,8 +66,10 @@ bool FieldTrialParamAssociator::GetFieldTrialParamsWithoutFallback(
 }
 
 void FieldTrialParamAssociator::ClearAllParamsForTesting() {
-  AutoLock scoped_lock(lock_);
-  field_trial_params_.clear();
+  {
+    AutoLock scoped_lock(lock_);
+    field_trial_params_.clear();
+  }
   FieldTrialList::ClearParamsFromSharedMemoryForTesting();
 }