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

base/critical_closure.h

Index: base/critical_closure.h
diff --git a/base/critical_closure.h b/base/critical_closure.h
index ca51ed5cef27d40923ba8e4d99de4044c63fa457..e6578479863279b651af9aefbf70fd4d493ef7e1 100644
--- a/base/critical_closure.h
+++ b/base/critical_closure.h
@@ -23,9 +23,11 @@ namespace base {
 // background running time, |MakeCriticalClosure| should be applied on them
 // before posting.
 #if defined(OS_IOS)
-base::Closure MakeCriticalClosure(const base::Closure& closure);
+template <typename CallbackType>
+CallbackType MakeCriticalClosure(const CallbackType& closure);

[gab, 2014/04/30 04:25:00]

Had to make this more generic to support use-case below; happy to rename
file/method if this is otherwise fine with you and owners of this file.

 #else
-inline base::Closure MakeCriticalClosure(const base::Closure& closure) {
+template <typename CallbackType>
+inline CallbackType MakeCriticalClosure(const CallbackType& 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;