Add APIs to protect critical tasks on iOS.

base/critical_closure.h

Index: base/critical_closure.h
diff --git a/base/critical_closure.h b/base/critical_closure.h
new file mode 100644
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--- /dev/null
+++ b/base/critical_closure.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_CRITICAL_CLOSURE_H_
+#define BASE_CRITICAL_CLOSURE_H_
+
+#include "base/callback.h"
+
+namespace base {
+
+// Returns a closure that will continue to run for a period of time when the
+// application goes to the background if possible on platforms where
+// applications don't execute while backgrounded, otherwise the original task is
+// returned.
+//
+// Example:
+//   file_message_loop_proxy_->PostTask(
+//       FROM_HERE,
+//       MakeCriticalClosure(base::Bind(&WriteToDiskTask, path_, data)));
+//
+// Note new closures might be posted in this closure. If the new closures need
+// background running time, |MakeCriticalClosure| should be applied on them
+// before posting.
+#if defined(OS_IOS)
+base::Closure MakeCriticalClosure(const base::Closure& closure);
+#else
+base::Closure MakeCriticalClosure(const base::Closure& closure) {
+  // No-op for platforms where the application does not need to acquire
+  // background time for closures to finish when it goes into the background.
+  return closure;
+}
+#endif  // !defined(OS_IOS)
+
+}  // namespace base
+
+#endif  // BASE_CRITICAL_CLOSURE_H_