Make ScopedFeatureList a no-op if Init*() is never called on it.

Previously, it would clobber state when going out of scope, which
is undesirable.

Bug: 854812
Change-Id: I2fd2859b28eed5e8247361da3b105068b348813b
Reviewed-on: https://chromium-review.googlesource.com/1112481Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569752}

base/test/scoped_feature_list.cc

@@ -85,10 +85,15 @@ void OverrideFeatures(const std::string& features,
 ScopedFeatureList::ScopedFeatureList() = default;
 
 ScopedFeatureList::~ScopedFeatureList() {
-  if (field_trial_override_)
+  // If one of the Init() functions was never called, don't reset anything.
+  if (!init_called_)
+    return;
+
+  if (field_trial_override_) {
     base::FieldTrialParamAssociator::GetInstance()->ClearParamsForTesting(
         field_trial_override_->trial_name(),
         field_trial_override_->group_name());
+  }
 
   FeatureList::ClearInstanceForTesting();
   if (original_feature_list_)
@@ -106,6 +111,7 @@ void ScopedFeatureList::InitWithFeatureList(
   DCHECK(!original_feature_list_);
   original_feature_list_ = FeatureList::ClearInstanceForTesting();
   FeatureList::SetInstance(std::move(feature_list));
+  init_called_ = true;
 }
 
 void ScopedFeatureList::InitFromCommandLine(

base/test/scoped_feature_list.h

@@ -110,6 +110,7 @@ class ScopedFeatureList final {
   void InitAndEnableFeatureWithFieldTrialOverride(const Feature& feature,
                                                   FieldTrial* trial);
 
+  bool init_called_ = false;
   std::unique_ptr<FeatureList> original_feature_list_;
   scoped_refptr<FieldTrial> field_trial_override_;
   std::unique_ptr<base::FieldTrialList> field_trial_list_;

base/test/scoped_feature_list_unittest.cc

@@ -293,5 +293,16 @@ TEST_F(ScopedFeatureListTest, FeatureOverrideKeepsOtherExistingDefaultFeature) {
   }
 }
 
+TEST_F(ScopedFeatureListTest, ScopedFeatureListIsNoopWhenNotInitialized) {
+  test::ScopedFeatureList feature_list1;
+  feature_list1.InitFromCommandLine("*TestFeature1", std::string());
+
+  // A ScopedFeatureList on which Init() is not called should not reset things
+  // when going out of scope.
+  { test::ScopedFeatureList feature_list2; }
+
+  ExpectFeatures("*TestFeature1", std::string());
+}
+
 }  // namespace test
 }  // namespace base