Make FieldTrialList usable after clearing VariationsParamsManager.

components/variations/variations_params_manager.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 "components/variations/variations_params_manager.h"
 
 #include <utility>
 
 #include "base/feature_list.h"
 #include "base/metrics/field_trial.h"
@@ skipping 68 matching lines @@
 
 void VariationParamsManager::ClearAllVariationIDs() {
   variations::testing::ClearAllVariationIDs();
 }
 
 void VariationParamsManager::ClearAllVariationParams() {
   variations::testing::ClearAllVariationParams();
   // When the scoped feature list is destroyed, it puts back the original
   // feature list that was there when InitWithFeatureList() was called.
   scoped_feature_list_.reset(new base::test::ScopedFeatureList());
+  // Ensure the destructor is called properly, so it can be freshly recreated.
+  field_trial_list_.reset();
+  field_trial_list_ = base::MakeUnique<base::FieldTrialList>(nullptr);
 }
 
 }  // namespace testing
 }  // namespace variations