Add a ScopedFeatureList class for testing and start using it.

components/ssl_config/BUILD.gn

 # 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.
 
 # GYP version: components/ssl_config.gypi:ssl_config
 static_library("ssl_config") {
   sources = [
     "ssl_config_prefs.cc",
     "ssl_config_prefs.h",
     "ssl_config_service_manager.h",
@@ skipping 12 matching lines @@
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "ssl_config_service_manager_pref_unittest.cc",
   ]
   deps = [
     ":ssl_config",
     "//base",
+    "//base/test:test_support",
     "//components/prefs:test_support",
     "//net",
     "//testing/gtest",
   ]
 }