Avoid changing enabled features while the browser is already running.

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 172 matching lines @@
   // 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);
 
+  // Test harnesses should disallow calls to ClearInstanceForTesting and
+  // RestoreInstanceForTesting from test code, while the test code is running in
+  // presence of other threads that make use of base::FeatureList (because such
+  // calls are inherently racey).
+  static void BlockOverridingInstanceForTesting(bool block);
+
  private:
   FRIEND_TEST_ALL_PREFIXES(FeatureListTest, CheckFeatureIdentity);
   FRIEND_TEST_ALL_PREFIXES(FeatureListTest,
                            StoreAndRetrieveFeaturesFromSharedMemory);
   FRIEND_TEST_ALL_PREFIXES(FeatureListTest,
                            StoreAndRetrieveAssociatedFeaturesFromSharedMemory);
 
   struct OverrideEntry {
     // The overridden enable (on/off) state of the feature.
     const OverrideState overridden_state;
@@ skipping 74 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_