Display storage.sync items alongside storage.local items in the extension storage inspector. Because storage.local and storage.sync can each contain an item with the same name, the item name alone is no longer unique. Use a compound uniqueKey consisting of the item name and storage area to disambiguate them within the same table. This approach follows the one used by the cookies storage actor. The Storage Inspector client automatically uses the first field returned by getFields() as the unique identifier for table rows, so exposing the compound uniqueKey there lets existing interfaces such as getValuesForHost(), editItem(), and removeItem() receive it without changes to their external signatures. Differential Revision: https://phabricator.services.mozilla.com/D304326