Add a ScopedFeatureList class for testing and start using it.

base/test/scoped_feature_list.h

Index: base/test/scoped_feature_list.h
diff --git a/base/test/scoped_feature_list.h b/base/test/scoped_feature_list.h
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index 0000000000000000000000000000000000000000..02603e1c1239bebe69468d050a10e4c851562785
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+++ b/base/test/scoped_feature_list.h
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+// 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();
+
+  // 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_