Basic UkmService implementation

ios/chrome/browser/metrics/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("metrics") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "field_trial_synchronizer.cc",
     "field_trial_synchronizer.h",
     "ios_chrome_metrics_service_accessor.cc",
     "ios_chrome_metrics_service_accessor.h",
     "ios_chrome_metrics_service_client.h",
     "ios_chrome_metrics_service_client.mm",
     "ios_chrome_metrics_services_manager_client.h",
     "ios_chrome_metrics_services_manager_client.mm",
     "ios_chrome_origins_seen_service_factory.cc",
     "ios_chrome_origins_seen_service_factory.h",
     "ios_chrome_stability_metrics_provider.cc",
     "ios_chrome_stability_metrics_provider.h",
     "mobile_session_shutdown_metrics_provider.h",
     "mobile_session_shutdown_metrics_provider.mm",
     "previous_session_info.h",
     "previous_session_info.mm",
     "previous_session_info_private.h",
   ]
   deps = [
     "//base",
     "//components/crash/core/common",
+    "//components/keyed_service/core",
     "//components/keyed_service/ios",
     "//components/metrics",
     "//components/metrics:net",
     "//components/metrics:profiler",
     "//components/metrics:ui",
     "//components/metrics_services_manager",
     "//components/navigation_metrics",
     "//components/omnibox/browser",
     "//components/prefs",
     "//components/rappor",
     "//components/signin/core/browser",
     "//components/sync",
     "//components/translate/core/browser",
+    "//components/ukm",
+    "//components/ukm:observers",
     "//components/variations",
     "//components/variations/service",
     "//components/version_info",
     "//ios/chrome/browser",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/crash_report",
     "//ios/chrome/browser/google",
+    "//ios/chrome/browser/history:history",
     "//ios/chrome/browser/signin",
     "//ios/chrome/browser/sync",
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/variations",
     "//ios/chrome/browser/variations:ios_chrome_ui_string_overrider_factory",
     "//ios/chrome/common",
     "//ios/web",
   ]
 }
 
@@ skipping 88 matching lines @@
     "//ios/chrome/browser/ui/tools_menu",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/testing:ios_test_support",
     "//ios/third_party/earl_grey",
     "//ios/web:test_support",
     "//ui/base",
   ]
   libs = [ "XCTest.framework" ]
 }