mac: Fix a dangling delegate bug with MCPopupController.

ui/message_center/cocoa/popup_controller.mm

Index: ui/message_center/cocoa/popup_controller.mm
diff --git a/ui/message_center/cocoa/popup_controller.mm b/ui/message_center/cocoa/popup_controller.mm
index 889d8dea6c590c78395fa032143ef54b77359899..13e30e4a3a20c10851eb734d901f23d7bbeecaa5 100644
--- a/ui/message_center/cocoa/popup_controller.mm
+++ b/ui/message_center/cocoa/popup_controller.mm
@@ -136,7 +136,17 @@
   return self;
 }
 
+#ifndef NDEBUG
+- (void)dealloc {
+  DCHECK(hasBeenClosed_);
+  [super dealloc];
+}
+#endif
+
 - (void)close {
+#ifndef NDEBUG
+  hasBeenClosed_ = YES;
+#endif
   [self setBoundsAnimation:nil];
   if (trackingArea_.get())
     [[[self window] contentView] removeTrackingArea:trackingArea_.get()];
@@ -239,6 +249,9 @@
   if (isClosing_)
     return;
 
+#ifndef NDEBUG
+  hasBeenClosed_ = YES;
+#endif
   isClosing_ = YES;
 
   // If the notification was swiped closed, do not animate it as the