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

ios/chrome/browser/ui/tab_switcher/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/tabswitcher_full_history.png",
     "resources/tabswitcher_full_history@2x.png",
     "resources/tabswitcher_full_history@3x.png",
     "resources/tabswitcher_incognito.png",
@@ skipping 32 matching lines @@
     "resources/tabswitcher_tablet.png",
     "resources/tabswitcher_tablet@2x.png",
     "resources/tabswitcher_tablet@3x.png",
   ]
   outputs = [
     "{{bundle_resources_dir}}/{{source_file_part}}",
   ]
 }
 
 source_set("tab_switcher") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "tab_model_snapshot.h",
     "tab_model_snapshot.mm",
     "tab_switcher.h",
     "tab_switcher_button.h",
     "tab_switcher_button.mm",
     "tab_switcher_cache.h",
     "tab_switcher_cache.mm",
     "tab_switcher_controller.h",
     "tab_switcher_controller.mm",
@@ skipping 131 matching lines @@
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/third_party/earl_grey",
     "//ui/base",
   ]
   libs = [
     "UIKit.framework",
     "XCTest.framework",
   ]
 }