Add APIs to protect critical tasks on iOS.

base/ios/scoped_critical_action.h

Index: base/ios/scoped_critical_action.h
diff --git a/base/ios/scoped_critical_action.h b/base/ios/scoped_critical_action.h
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+++ b/base/ios/scoped_critical_action.h
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+// Copyright (c) 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 BASE_IOS_SCOPED_CRITICAL_ACTION_H_
+#define BASE_IOS_SCOPED_CRITICAL_ACTION_H_
+
+#include "base/synchronization/lock.h"
+
+namespace base {
+namespace ios {
+
+// This class attempts to allow the application to continue to run for a period
+// of time after it transitions to the background. The construction of an
+// instance of this class marks the beginning of a task that needs background
+// running time when the application is moved to the background and the
+// destruction marks the end of such a task.
+//
+// Note there is no guarantee that the task will continue to finish when the
+// application is moved to the background.
+//
+// This class should be used at times where leaving a task unfinished might be
+// detrimental to user experience. For example, it should be used to ensure that
+// the application has enough time to save important data or at least attempt to
+// save such data.
+class ScopedCriticalAction {
+ public:
+  ScopedCriticalAction();
+  ~ScopedCriticalAction();
+
+ private:
+  // Informs the OS that the background task has completed.
+  void EndBackgroundTask();
+  // |UIBackgroundTaskIdentifier| returned by
+  // |beginBackgroundTaskWithExpirationHandler:| when marking the beginning of
+  // a long-running background task. It is defined as an |unsigned int| instead
+  // of a |UIBackgroundTaskIdentifier| so this class can be used in .cc files.
+  unsigned int background_task_id_;
+  Lock background_task_id_lock_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedCriticalAction);
+};
+
+}  // namespace ios
+}  // namespace base
+
+#endif  // BASE_IOS_SCOPED_CRITICAL_ACTION_H_