[ObjC ARC] Converts ios/chrome/browser/metrics:metrics_internal to ARC.

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",
@@ skipping 65 matching lines @@
     "//components/metrics:test_support",
     "//components/prefs",
     "//components/prefs:test_support",
     "//components/version_info",
     "//ios/chrome/test:test_support",
     "//testing/gtest",
   ]
 }
 
 source_set("metrics_internal") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "first_user_action_recorder.cc",
     "first_user_action_recorder.h",
     "new_tab_page_uma.h",
     "new_tab_page_uma.mm",
     "size_class_recorder.h",
     "size_class_recorder.mm",
     "size_class_recorder_private.h",
     "tab_usage_recorder.h",
     "tab_usage_recorder.mm",
@@ skipping 73 matching lines @@
     "//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:earl_grey_test_support",
     "//ios/web:test_support",
     "//ui/base",
   ]
   libs = [ "XCTest.framework" ]
 }