Prefer ScopedNestableTaskAllower over manual save/restore

content/browser/download/drag_download_file.cc

Index: content/browser/download/drag_download_file.cc
diff --git a/content/browser/download/drag_download_file.cc b/content/browser/download/drag_download_file.cc
index cecb9be189225e4982ab684f237322a1fbe8aba9..93adf6d248699c73628fa117fd0383f55f5aae29 100644
--- a/content/browser/download/drag_download_file.cc
+++ b/content/browser/download/drag_download_file.cc
@@ -223,11 +223,9 @@ void DragDownloadFile::AssertCurrentlyOnUIThread() {
 void DragDownloadFile::StartNestedMessageLoop() {
   AssertCurrentlyOnDragThread();
 
-  bool old_state = MessageLoop::current()->NestableTasksAllowed();
-  MessageLoop::current()->SetNestableTasksAllowed(true);
+  MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
   is_running_nested_message_loop_ = true;
   MessageLoop::current()->Run();

[jar (doing other things), 2012/02/11 03:24:38]

This looks suspiciously like a bug... but I'm not sure.

I suspect we should have line 229 that does:
is_running_nested_message_loop_ = false.

without it, I'm wary that a call in line 234 will post a spurious Quit()
message.


Alternatively, I'd be OK with asserting (here) that it was set to false via line
235.  ...but looking at the logic, I'm confused about whether it is evyr
possible to reach lien 234 without having is_running_nested_message_loop_ .  A
brief read of the code (perhaps too brief) suggests that 231 is only called when
in a nested message loop... but I'm not sure at all.

[dhollowa, 2012/02/13 17:44:26]

Done.  I've added DCHECK to verify the logic.  If this trips we might consider
using |MessageLoop::IsNested| instead.

-  MessageLoop::current()->SetNestableTasksAllowed(old_state);
 }
 
 void DragDownloadFile::QuitNestedMessageLoop() {