Allow overriding variation parameter via chrome://flags.

components/flags_ui/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.
 
 source_set("flags_ui") {
   sources = [
     "feature_entry.cc",
     "feature_entry.h",
     "feature_entry_macros.h",
     "flags_state.cc",
     "flags_state.h",
     "flags_storage.h",
     "flags_ui_constants.cc",
     "flags_ui_constants.h",
     "flags_ui_pref_names.cc",
     "flags_ui_pref_names.h",
     "pref_service_flags_storage.cc",
     "pref_service_flags_storage.h",
   ]
 
   deps = [
     ":switches",
     "//base",
     "//components/pref_registry",
     "//components/prefs",
     "//components/strings",
+    "//components/variations",
     "//ui/base",
   ]
 }
 
 # This is a separate target so that the dependencies of
 # //chrome/common can be kept minimal.
 source_set("switches") {
   sources = [
     "flags_ui_switches.cc",
     "flags_ui_switches.h",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "flags_state_unittest.cc",
   ]
 
   deps = [
     ":flags_ui",
     ":switches",
     "//base",
     "//components/prefs:test_support",
     "//components/strings",
+    "//components/variations",
     "//testing/gtest",
   ]
 }