Make FieldTrialList usable after clearing VariationsParamsManager.

components/variations/variations_params_manager.cc

Index: components/variations/variations_params_manager.cc
diff --git a/components/variations/variations_params_manager.cc b/components/variations/variations_params_manager.cc
index e4b92cc1bbbad950f2bda1c8763a3d61b36514a4..4d90a102e0d642c2cecbce564a921758a2abc0b6 100644
--- a/components/variations/variations_params_manager.cc
+++ b/components/variations/variations_params_manager.cc
@@ -86,6 +86,9 @@ void VariationParamsManager::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