Add APIs to protect critical tasks on iOS.

base/critical_closure_stub.cc

Index: base/critical_closure_stub.cc
diff --git a/base/critical_closure_stub.cc b/base/critical_closure_stub.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a2fd3365baa8c78595cef91ca83f9236170277c3
--- /dev/null
+++ b/base/critical_closure_stub.cc
@@ -0,0 +1,11 @@
+// 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.
+
+#include "base/critical_closure.h"
+
+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;
+}