[IndexedDB] Ensure that strong references to newly created cursors are

kept until the DOM Binding is created.

This fixes random crashes on websites that use IndexedDB cursors.
See also BZ bug 1599420

dom/indexedDB/ActorsChild.cpp

@@ -3291,6 +3291,10 @@ BackgroundCursorChild::HandleResponse(
   auto& responses =
     const_cast<nsTArray<ObjectStoreCursorResponse>&>(aResponses);
 
+  // If a new cursor is created, we need to keep a reference to it until the
+  // ResultHelper creates a DOM Binding.
+  RefPtr<IDBCursor> newCursor;
+
   for (ObjectStoreCursorResponse& response : responses) {
     StructuredCloneReadInfo cloneReadInfo(Move(response.cloneInfo()));
     cloneReadInfo.mDatabase = mTransaction->Database();
@@ -3300,8 +3304,6 @@ BackgroundCursorChild::HandleResponse(
                                     nullptr,
                                     cloneReadInfo.mFiles);
 
-    RefPtr<IDBCursor> newCursor;
-
     if (mCursor) {
       mCursor->Reset(Move(response.key()), Move(cloneReadInfo));
     } else {