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..cea6e7af0d086f5eba2c12471cbbcb0ea81e91bd 100644
--- a/base/critical_closure.h
+++ b/base/critical_closure.h
@@ -9,7 +9,8 @@
 
 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.
@@ -23,9 +24,11 @@ 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 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;