[ObjC ARC] Converts ios/chrome/browser/sessions:test_support to ARC.

ios/chrome/browser/sessions/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("sessions") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "ios_chrome_session_tab_helper.h",
     "ios_chrome_session_tab_helper.mm",
     "ios_chrome_tab_restore_service_client.h",
@@ skipping 41 matching lines @@
     "//components/sessions",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/tabs",
     "//ios/web",
     "//net",
   ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("test_support") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "test_session_service.h",
     "test_session_service.mm",
   ]
   deps = [
     ":sessions",
   ]
 }
 
@@ skipping 36 matching lines @@
   testonly = true
   sources = [
     "//ios/chrome/test/data/sessions/corrupted.plist",
     "//ios/chrome/test/data/sessions/session_m57.plist",
     "//ios/chrome/test/data/sessions/session_m58.plist",
   ]
   outputs = [
     "{{bundle_resources_dir}}/ios/chrome/test/data/sessions/{{source_file_part}}",
   ]
 }