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

ios/chrome/browser/ui/sync/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.
 
 import("//ios/public/provider/chrome/browser/build_config.gni")
 
 source_set("sync") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "sync_error_infobar_delegate.h",
     "sync_error_infobar_delegate.mm",
     "sync_util.h",
     "sync_util.mm",
   ]
   deps = [
     "//base",
     "//components/browser_sync",
     "//components/infobars/core",
@@ skipping 34 matching lines @@
     "//ios/public/provider/chrome/browser/signin:test_support",
     "//ios/testing:ios_test_support",
     "//ios/third_party/earl_grey",
     "//ios/web/public/test/http_server",
     "//net",
     "//ui/base",
     "//url",
   ]
   libs = [ "XCTest.framework" ]
 }