Bug 1291384 - 1. Take BrowserDB out of GeckoProfile; r=nalexander

Remove references to BrowserDB and its factory from GeckoProfile.
Instead of keeping the DB in mDB, GeckoProfile now keeps an arbitrary
object in mData. Using a data object lets us avoid needing another map
to map profiles to DBs. This feature could be very useful for GeckoView
consumers as well.

The new way to get a BrowserDB from a profile/context is through
BrowserDB.from(Context) or BrowserDB.from(GeckoProfile), which takes
care of creating a local DB if necessary and associating the DB with the
profile.