fetch/idb_store.cpp and fetch/idb_delete.cpp are currently failing due to IDB database only existing on main thread

[sbc100]

These tests have been disabled for a while and while attempting to re-enable them I ran into an issue where the IDB database doesn't exist on the proxied main thread (only on the main browser thread). I can't see how this was any ever different.

[mere-human]

I stumbled upon the same problem while trying stuff. It seems This like Fetch.staticInit() is called only from the main thread so Fetch.dbInstance is not initialised in other threads.