[Sync] Create iOS factory for UserEventService.

ios/chrome/browser/sync/BUILD.gn

 # 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.
 
 source_set("sync") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "ios_chrome_profile_sync_service_factory.cc",
     "ios_chrome_profile_sync_service_factory.h",
     "ios_chrome_sync_client.h",
     "ios_chrome_sync_client.mm",
     "ios_chrome_synced_tab_delegate.h",
     "ios_chrome_synced_tab_delegate.mm",
+    "ios_user_event_service_factory.cc",
+    "ios_user_event_service_factory.h",
     "sync_observer_bridge.h",
     "sync_observer_bridge.mm",
     "sync_setup_service.cc",
     "sync_setup_service.h",
     "sync_setup_service_factory.cc",
     "sync_setup_service_factory.h",
   ]
   deps = [
     "//base",
     "//components/autofill/core/browser",
@@ skipping 87 matching lines @@
   deps = [
     ":sync",
     "//base",
     "//components/browser_sync",
     "//components/sync",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/web/public/test",
     "//testing/gtest",
   ]
 }