[ObjC ARC] Converts ios/chrome/browser/ui/tabs:tabs to ARC.

ios/chrome/browser/ui/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.
 
 bundle_data("resources") {
   sources = [
     "resources/tabstrip_background_tab@2x~ipad.png",
     "resources/tabstrip_background_tab~ipad.png",
     "resources/tabstrip_foreground_tab@2x~ipad.png",
     "resources/tabstrip_foreground_tab~ipad.png",
@@ skipping 20 matching lines @@
     "resources/tabstrip_toggle_button_gradient@2x~ipad.png",
     "resources/tabstrip_toggle_button_gradient@3x~ipad.png",
     "resources/tabstrip_toggle_button_gradient~ipad.png",
   ]
   outputs = [
     "{{bundle_resources_dir}}/{{source_file_part}}",
   ]
 }
 
 source_set("tabs") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "tab_strip_controller+tab_switcher_animation.h",
     "tab_strip_controller.h",
     "tab_strip_controller.mm",
     "tab_strip_controller_private.h",
     "tab_strip_view.h",
     "tab_strip_view.mm",
     "tab_view.h",
     "tab_view.mm",
     "target_frame_cache.h",
@@ skipping 52 matching lines @@
     "//ios/chrome/app/strings",
     "//ios/chrome/browser",
     "//ios/chrome/browser/ui",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/third_party/earl_grey",
     "//ui/base",
   ]
   libs = [ "XCTest.framework" ]
 }