[ObjC ARC] Converts ios/chrome/test/earl_grey:hooks to ARC.

ios/chrome/test/earl_grey/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/chrome/test/earl_grey/chrome_ios_eg_test.gni")
 
 group("all_tests") {
   testonly = true
   deps = [
     ":ios_chrome_device_check_egtests",
@@ skipping 184 matching lines @@
   ]
 
   libs = [
     "OCHamcrest.framework",
     "WebKit.framework",
     "XCTest.framework",
   ]
 }
 
 source_set("hooks") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "eg_tests_hook.mm",
   ]
   deps = [
     "//ios/chrome/app:tests_hook",
   ]
 }