[ObjC ARC] Converts ios/public/provider/chrome/browser/ui:ui to ARC.

ios/public/provider/chrome/browser/ui/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("ui") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "app_rating_prompt.h",
     "default_ios_web_view_factory.h",
     "default_ios_web_view_factory.mm",
     "logo_vendor.h",
     "text_field_styling.h",
   ]
   deps = [
     "//base",
     "//ios/public/provider/chrome/browser/voice",
   ]
   libs = [ "CoreGraphics.framework" ]
 }
 
 source_set("test_support") {
   testonly = true
   sources = [
     "test_styled_text_field.h",
     "test_styled_text_field.mm",
   ]
   deps = [
     ":ui",
   ]
 }