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

ios/chrome/browser/snapshots/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("snapshots") {
   sources = [
     "lru_cache.h",
     "lru_cache.mm",
     "snapshot_cache.h",
     "snapshot_cache.mm",
@@ skipping 18 matching lines @@
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/ui",
     "//ios/chrome/browser/web_state_list",
     "//ios/web",
   ]
   libs = [ "QuartzCore.framework" ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "lru_cache_unittest.mm",
     "snapshot_cache_unittest.mm",
     "snapshots_util_unittest.mm",
   ]
   deps = [
     ":snapshots",
     "//base",
     "//ios/chrome/browser/ui",
@@ skipping 12 matching lines @@
   ]
   deps = [
     ":snapshots",
     "//base",
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/ui",
     "//ios/web",
   ]
   libs = [ "UIKit.framework" ]
 }