Add a ScopedFeatureList class for testing and start using it.

components/variations/service/variations_service_unittest.cc

Index: components/variations/service/variations_service_unittest.cc
diff --git a/components/variations/service/variations_service_unittest.cc b/components/variations/service/variations_service_unittest.cc
index 6abfc8a8f5c16297da3c3e6bf97d86d5037dca91..6530849ab6c129381691694f85353d6fd287f923 100644
--- a/components/variations/service/variations_service_unittest.cc
+++ b/components/variations/service/variations_service_unittest.cc
@@ -272,10 +272,6 @@ TEST_F(VariationsServiceTest, CreateTrialsFromSeed) {
   TestingPrefServiceSimple prefs;
   VariationsService::RegisterPrefs(prefs.registry());
 
-  // Setup base::FeatureList.
-  base::FeatureList::ClearInstanceForTesting();
-  base::FeatureList::SetInstance(base::WrapUnique(new base::FeatureList()));
-
   // Create a local base::FieldTrialList, to hold the field trials created in
   // this test.
   base::FieldTrialList field_trial_list(nullptr);
@@ -303,10 +299,6 @@ TEST_F(VariationsServiceTest, CreateTrialsFromSeedNoLastFetchTime) {
   TestingPrefServiceSimple prefs;
   VariationsService::RegisterPrefs(prefs.registry());
 
-  // Setup base::FeatureList.
-  base::FeatureList::ClearInstanceForTesting();
-  base::FeatureList::SetInstance(base::WrapUnique(new base::FeatureList()));
-
   // Create a local base::FieldTrialList, to hold the field trials created in
   // this test.
   base::FieldTrialList field_trial_list(nullptr);
@@ -334,10 +326,6 @@ TEST_F(VariationsServiceTest, CreateTrialsFromOutdatedSeed) {
   TestingPrefServiceSimple prefs;
   VariationsService::RegisterPrefs(prefs.registry());
 
-  // Setup base::FeatureList.
-  base::FeatureList::ClearInstanceForTesting();
-  base::FeatureList::SetInstance(base::WrapUnique(new base::FeatureList()));
-
   // Create a local base::FieldTrialList, to hold the field trials created in
   // this test.
   base::FieldTrialList field_trial_list(nullptr);