Add a ScopedFeatureList class for testing and start using it.

base/test/scoped_feature_list.h

+// Copyright 2016 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_TEST_SCOPED_FEATURE_LIST_H_
+#define BASE_TEST_SCOPED_FEATURE_LIST_H_
+
+#include "base/feature_list.h"
+
+namespace base {
+namespace test {
+
+// ScopedFeatureList resets the global FeatureList instance to a new empty
+// instance and restores the original instance upon destruction.
+// Note: Re-using the same object is not allowed. To reset the feature
+// list and initialize it anew, destroy an existing scoped list and init
+// a new one.
+class ScopedFeatureList final {
+ public:
+  ScopedFeatureList();
+  ~ScopedFeatureList();
+
+  // Initializes and registers a FeatureList instance with no overrides.
+  void Init();

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

Optional nit: I think it would be cleaner if each of the Init methods were a
constructor -- that way, code doesn't need to first construct the object, and
then init it.

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

That doesn't fit some use cases - where e.g. the scoped feature list is a member
of a test harness class and then *some* tests but not all initialize it to some
given state. I'll leave it as-is for now, but we can consider also adding the
ctor params as a short hand syntax for not calling the Init functions.

[Ilya Sherman, 2016/08/09 06:15:12]

FWIW, my preferred solution to that would be for the individual tests to declare
the scoped feature list, rather than having it be a member of the test harness
class.

[Alexei Svitkine (slow), 2016/08/09 15:27:43]

Okay, I can take a look at it when I plan to do my second pass in converting
existing tests, probably when I'll be traveling Thursday next week.

+
+  // Initializes and registers the given FeatureList instance.
+  void InitWithFeatureList(std::unique_ptr<FeatureList> feature_list);
+
+  // Initializes and registers a FeatureList instance with the given
+  // enabled and disabled features (comma-separated names).
+  void InitFromCommandLine(const std::string& enable_features,
+                           const std::string& disable_features);
+
+  // Initializes and registers a FeatureList instance enabling a single
+  // feature.
+  void InitAndEnableFeature(const base::Feature& feature);
+
+  // Initializes and registers a FeatureList instance disabling a single
+  // feature.
+  void InitAndDisableFeature(const base::Feature& feature);
+
+ private:
+  std::unique_ptr<FeatureList> original_feature_list_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedFeatureList);
+};
+
+}  // namespace test
+}  // namespace base
+
+#endif  // BASE_TEST_SCOPED_FEATURE_LIST_H_