As we convert the RevisionRecord to using Authority,
we no longer need Title instances, so we can convert
that to PageIdentity.
Ideally, we'd part away from using Title at all, but:
1. For foreign wikis PageIdentity has stronger validation,
so calling PageIdentity getId() on Title will break things.
There's still a lot of code depending on lax Title guarantees,
so we keep it.
2. A lot of code still depends on Title, so we try to pass it
through even if we don't nesessarily need to, to save cost
on recreating it later on.
Bug: T271458
Depends-On: I287400b967b467ea18bebbb579e881a785a19158
Change-Id: I63d9807264d7e2295afef51fc9d982447f92fcbd
