[ObjC ARC] Converts ios/chrome/browser/device_sharing:unit_tests to ARC.

ios/chrome/browser/device_sharing/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("device_sharing") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "device_sharing_manager.h",
     "device_sharing_manager.mm",
   ]
   deps = [
     "//base",
     "//components/handoff",
     "//components/prefs",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/prefs",
     "//net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "device_sharing_manager_unittest.mm",
   ]
   deps = [
     ":device_sharing",
     "//base",
     "//components/handoff",
     "//components/sync_preferences:test_support",
     "//ios/chrome/browser/browser_state:test_support",
@@ skipping 15 matching lines @@
     "//ios/chrome/browser/ui",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/third_party/earl_grey",
     "//ios/web:test_support",
     "//net",
     "//url",
   ]
   libs = [ "XCTest.framework" ]
 }