[ObjC ARC] Converts ios/chrome/app/application_delegate to ARC.

ios/chrome/app/application_delegate/memory_warning_helper.mm

 // 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.
 
 #import "ios/chrome/app/application_delegate/memory_warning_helper.h"
 
 #include "base/memory/memory_pressure_listener.h"
 #include "base/metrics/histogram.h"
 #include "ios/chrome/browser/crash_report/breakpad_helper.h"
 #import "ios/chrome/browser/metrics/previous_session_info.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 // The number of seconds to wait after a memory warning to clear the flag used
 // to detect Out Of Memory crashes.
 // NOTE: From local tests on various devices, this interval ranges between 1 and
 // 3 seconds. It is set to 5 to ensure all out of memory crashes are identified,
 // even if this may lead to overcounting them.
 const CFTimeInterval kOutOfMemoryResetTimeInterval = 5;
 }
 
 @interface MemoryWarningHelper () {
@@ skipping 38 matching lines @@
   [[PreviousSessionInfo sharedInstance] resetMemoryWarningFlag];
   breakpad_helper::SetMemoryWarningInProgress(false);
 }
 
 - (void)resetForegroundMemoryWarningCount {
   _foregroundMemoryWarningCount = 0;
   breakpad_helper::SetMemoryWarningCount(0);
 }
 
 @end