Bug 1917477 - LSNG: Make sure Promise objects are always released on the right thread; r=dom-storage-reviewers,asuth

Differential Revision: https://phabricator.services.mozilla.com/D219841

dom/localstorage/LocalStorageManager2.cpp

@@ -511,6 +511,7 @@ void AsyncRequestHelper::Finish() {
   }
 
   mManager = nullptr;
+  mPromise = nullptr;
 
   mState = State::Complete;
 }