Avoid changing enabled features while the browser is already running.

base/feature_list.h

Index: base/feature_list.h
diff --git a/base/feature_list.h b/base/feature_list.h
index c9f4a7b0c462805dfd622ef68317634bae251956..453deb32d0a1c0eeb0f02052102ce29fea3d7190 100644
--- a/base/feature_list.h
+++ b/base/feature_list.h
@@ -190,6 +190,12 @@ class BASE_EXPORT FeatureList {
   // 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,