Redirect HistoryService thread to TaskScheduler via a field trial.

components/history/core/browser/BUILD.gn

 # Copyright 2014 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.
 
 static_library("browser") {
   sources = [
     "delete_directive_handler.cc",
     "delete_directive_handler.h",
     "download_constants.h",
     "download_database.cc",
@@ skipping 82 matching lines @@
     "//base",
     "//base:i18n",
     "//components/favicon_base",
     "//components/history/core/common",
     "//components/keyed_service/core",
     "//components/prefs",
     "//components/query_parser",
     "//components/signin/core/browser",
     "//components/sync",
     "//components/url_formatter",
+    "//components/variations",
     "//components/version_info",
     "//google_apis",
     "//net",
     "//sql",
     "//third_party/sqlite",
     "//ui/base",
     "//ui/gfx",
     "//url",
   ]
 
@@ skipping 99 matching lines @@
     "//sql:test_support",
     "//testing/gtest",
     "//ui/gfx",
     "//ui/gfx:test_support",
     "//url",
   ]
   if (is_android) {
     sources += [ "android/android_history_types_unittest.cc" ]
   }
 }