Add a ScopedFeatureList class for testing and start using it.

base/feature_list.h

Index: base/feature_list.h
diff --git a/base/feature_list.h b/base/feature_list.h
index e9ed00a124ad8f701d05391ede500e0c5f84138b..698ecd533423e06c85a6748bd7d85ab23a46fb96 100644
--- a/base/feature_list.h
+++ b/base/feature_list.h
@@ -165,8 +165,16 @@ class BASE_EXPORT FeatureList {
   // process. This should only be called once and |instance| must not be null.
   static void SetInstance(std::unique_ptr<FeatureList> instance);
 
-  // Clears the previously-registered singleton instance for tests.
-  static void ClearInstanceForTesting();
+  // Clears the previously-registered singleton instance for tests and returns
+  // the old instance.
+  // Note: Most tests should never call this directly. Instead consider using
+  // base::test::ScopedFeatureList.
+  static std::unique_ptr<FeatureList> ClearInstanceForTesting();
+
+  // Sets a given (initialized) |instance| to be the singleton feature list,
+  // for testing. Existing instance must be null. This is primarily intended
+  // to support base::test::ScopedFeatureList helper class.
+  static void RestoreInstanceForTesting(std::unique_ptr<FeatureList> instance);
 
  private:
   FRIEND_TEST_ALL_PREFIXES(FeatureListTest, CheckFeatureIdentity);