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

ios/chrome/test/ocmock/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("ocmock") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "OCMockObject+BreakpadControllerTesting.h",
     "OCMockObject+BreakpadControllerTesting.mm",
   ]
   deps = [
     "//base",
     "//breakpad:client",
     "//testing/gtest",
     "//third_party/ocmock",
   ]
 }