Add a ScopedFeatureList class for testing and start using it.

base/feature_list.h

 // Copyright 2015 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.
 
 #ifndef BASE_FEATURE_LIST_H_
 #define BASE_FEATURE_LIST_H_
 
 #include <map>
 #include <memory>
 #include <string>
@@ skipping 147 matching lines @@
                                  const std::string& disable_features);
 
   // Returns the singleton instance of FeatureList. Will return null until an
   // instance is registered via SetInstance().
   static FeatureList* GetInstance();
 
   // Registers the given |instance| to be the singleton feature list for this
   // 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

[Ilya Sherman, 2016/08/09 01:06:57]

nit: s/directly, instead/directly. Instead

[Alexei Svitkine (slow), 2016/08/09 01:26:16]

Done.

+  // base::test::ScopedFeatureList.
+  static std::unique_ptr<FeatureList> ClearInstanceForTesting();
+
+  // Sets an existing (initialized) |instance| to be the singleton feature
+  // list, for testing. Existing instance must be null. This is primarily

[Ilya Sherman, 2016/08/09 01:06:57]

nit: You use the word "existing" in two different ways here -- please rephrase
one of them.

[Alexei Svitkine (slow), 2016/08/09 01:26:16]

Done.

+  // intended to support base::test::ScopedFeatureList helper class.
+  static void RestoreInstanceForTesting(std::unique_ptr<FeatureList> instance);
 
  private:
   FRIEND_TEST_ALL_PREFIXES(FeatureListTest, CheckFeatureIdentity);
 
   struct OverrideEntry {
     // The overridden enable (on/off) state of the feature.
     const OverrideState overridden_state;
 
     // An optional associated field trial, which will be activated when the
     // state of the feature is queried for the first time. Weak pointer to the
@@ skipping 71 matching lines @@
 
   // Whether this object has been initialized from command line.
   bool initialized_from_command_line_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(FeatureList);
 };
 
 }  // namespace base
 
 #endif  // BASE_FEATURE_LIST_H_