Revert of Remove Enable Exits On Suspend flags on iOS

ios/chrome/app/UIApplication+ExitsOnSuspend.h

+// Copyright 2012 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.
+
+#ifndef IOS_CHROME_APP_UIAPPLICATION_EXITSONSUSPEND_H_
+#define IOS_CHROME_APP_UIAPPLICATION_EXITSONSUSPEND_H_
+
+#if !defined(NDEBUG)
+
+#import <UIKit/UIKit.h>
+
+// Key in the UserDefaults for toggling exit on suspend.
+extern NSString* const kExitsOnSuspend;
+
+// WARNING: This is intended for non AppStore builds, since it's not allowed to
+// quit an app programmatically. Internally we call exit(0).
+@interface UIApplication (ExitsOnSuspend)
+
+// When the app goes in background, terminate the app when all background tasks
+// have finished. Use this in -applicationDidEnterBackground:.
+- (void)cr_terminateWhenDoneWithBackgroundTasks;
+
+// Cancel termination. Use this in -applicationWillEnterForeground:.
+- (void)cr_cancelTermination;
+
+@end
+
+#endif  // !defined(NDEBUG)
+
+#endif  // IOS_CHROME_APP_UIAPPLICATION_EXITSONSUSPEND_H_