Bug 1477767 - disconnect request when XPCOM is going to shutdown. r=cpearce

The reason we hit this assertion is that we still connected to Then() and waited for its result when the resolve or reject
runnable which dispatched by ThenValue can't be executed because the target thread had been shutdown.

Therefore, when XPCOM is going to shutdown, we should disconnect the Then() because it might not have a chance to execute
its resolve/reject method.

Differential Revision: https://phabricator.services.mozilla.com/D5893

dom/html/HTMLMediaElement.cpp

@@ -6798,6 +6798,9 @@ void
 HTMLMediaElement::NotifyShutdownEvent()
 {
   mShuttingDown = true;
+  // Since target thread had been shutdown, it's no chance to execute the Then()
+  // afterward. Therefore, we should disconnect the request.
+  mAutoplayPermissionRequest.DisconnectIfExists();
   ResetState();
   AddRemoveSelfReference();
 }