Prefer ScopedNestableTaskAllower over manual save/restore

ipc/ipc_sync_channel.cc

 // 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 "ipc/ipc_sync_channel.h"
 
 #include "base/bind.h"
 #include "base/lazy_instance.h"
 #include "base/location.h"
 #include "base/logging.h"
@@ skipping 488 matching lines @@
   // outermost call completes first, etc.
   if (old_send_done_event_watcher) {
     old_delegate = old_send_done_event_watcher->delegate();
     old_event = old_send_done_event_watcher->GetWatchedEvent();
     old_send_done_event_watcher->StopWatching();
   }
 
   sync_msg_queue->set_top_send_done_watcher(&send_done_watcher);
 
   send_done_watcher.StartWatching(context->GetSendDoneEvent(), context);
-  bool old_state = MessageLoop::current()->NestableTasksAllowed();
 
-  MessageLoop::current()->SetNestableTasksAllowed(true);
-  MessageLoop::current()->Run();
-  MessageLoop::current()->SetNestableTasksAllowed(old_state);
+  {
+    MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
+    MessageLoop::current()->Run();
+  }
 
   sync_msg_queue->set_top_send_done_watcher(old_send_done_event_watcher);
   if (old_send_done_event_watcher && old_event) {
     old_send_done_event_watcher->StartWatching(old_event, old_delegate);
   }
 }
 
 void SyncChannel::OnWaitableEventSignaled(WaitableEvent* event) {
   DCHECK(event == sync_context()->GetDispatchEvent());
   // The call to DispatchMessages might delete this object, so reregister
   // the object watcher first.
   event->Reset();
   dispatch_watcher_.StartWatching(event, this);
   sync_context()->DispatchMessages();
 }
 
 void SyncChannel::StartWatching() {
   // Ideally we only want to watch this object when running a nested message
   // loop.  However, we don't know when it exits if there's another nested
   // message loop running under it or not, so we wouldn't know whether to
   // stop or keep watching.  So we always watch it, and create the event as
   // manual reset since the object watcher might otherwise reset the event
   // when we're doing a WaitMany.
   dispatch_watcher_.StartWatching(sync_context()->GetDispatchEvent(), this);
 }
 
 }  // namespace IPC