Move setup of base::test::ScopedFeatureList earlier, to avoid data race.

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 <initializer_list>
 
 #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.
+//
+// ScopedFeatureList needs to be initialized (via one of Init... methods)
+// before running code that inspects the state of features.  In practice this
+// means:
+// - In browser tests, one of Init... methods should be called from the
+//   overriden ::testing::Test::SetUp method. For example:
+//     void SetUp() override {
+//       scoped_feature_list_.InitAndEnableFeature(features::kMyFeatureHere);
+//       InProcessBrowserTest::SetUp();
+//     }
 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);
@@ skipping 20 matching lines @@
  private:
   std::unique_ptr<FeatureList> original_feature_list_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedFeatureList);
 };
 
 }  // namespace test
 }  // namespace base
 
 #endif  // BASE_TEST_SCOPED_FEATURE_LIST_H_