When about:url-classifier ask cache information by using |GetCacheInfo| API, this API
will send a SYNC request to off-main thread. If NSS module is not initialized then off-main thread
will send init request to main thread, and this causes a deadlock.
This patch change |GetCacheInfo| API to asynchronous so main thread will not be
blocked.
MozReview-Commit-ID: 3y2hOjCoRUj
