[ios] Convert //ios/chrome/browser/tabs target to ARC.

ios/chrome/browser/tabs/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("tabs") {
   sources = [
     "legacy_tab_helper.h",
     "tab.h",
     "tab_delegate.h",
     "tab_dialog_delegate.h",
@@ skipping 13 matching lines @@
   deps = [
     "//components/sessions",
     "//components/signin/ios/browser",
     "//components/sync_sessions",
     "//ios/chrome/browser/web:sad_tab_tab_helper_delegate",
     "//ios/net",
     "//ios/web:user_agent",
     "//ui/base",
   ]
   libs = [ "UIKit.framework" ]
+  configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 source_set("tabs_internal") {
   sources = [
     "legacy_tab_helper.mm",
     "tab.h",
     "tab.mm",
     "tab_helper_util.mm",
     "tab_model.mm",
     "tab_model_closing_web_state_observer.h",
@@ skipping 141 matching lines @@
     ":tabs",
     ":tabs_internal",
     "//base",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/sessions:test_support",
     "//ios/web:test_support",
     "//testing/gtest",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }