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

ios/chrome/browser/memory/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("memory") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "memory_debugger.h",
     "memory_debugger.mm",
     "memory_debugger_manager.h",
     "memory_debugger_manager.mm",
     "memory_metrics.cc",
     "memory_metrics.h",
   ]
   deps = [
     "//base",
     "//components/prefs",
     "//ios/chrome/browser",
     "//ios/chrome/browser/ui",
   ]
 }