Introduce a new framework for back-and-forth tracked/protected preferences migration.

base/critical_closure.h

 // 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
+// Returns a closure (which may return a result, but must not require any extra
+// arguments) 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);
+template <typename R>
+Callback<R(void)> MakeCriticalClosure(const Callback<R(void)>& closure);
 #else
-inline base::Closure MakeCriticalClosure(const base::Closure& closure) {
+template <typename R>
+inline Callback<R(void)> MakeCriticalClosure(const Callback<R(void)>& 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_