This shifts responsibility of constructing a VisitType for any given visit on the
caller of HistoryTrackingDelegate implementation, which is normally the engine.

This is generally what we want: engine is normally best suited to determine a visit type
for a URI that's currently being opened.

This is the preferred way to make sure our persisted account state
is up-to-date.
In the future, various parts of FxA (like SendTab) may mutate
account's internal state, and so it's necessary to make sure we
persist account changes as they happen.

Before, this happened manually after account creation.
Now persistence process is entirely callback-driven.