app-indexeddb-mirror and already persisted store

[eskan]

Description

Hi,
when the app-indexeddb-mirror is initialized, it's not populated with store data if it exists but remains empty, so change to livedata override current store.
test is done with Polymer 2.0 and persist storage is requested : navigator.storage.persist()

Expected outcome

app-indexeddb-mirror persistedData should be populate with stored value

Steps to reproduce

this is a sample app to test behavior
https://github.com/eskan/indexeddb-mirror-persited-test

Browsers Affected

• Chrome
• Firefox
• Safari 9
• Safari 8
• Safari 7
• Edge
• IE 11
• IE 10