[DMC #5.55v2] Add DocumentMarkerList::MarkerType()

Add DocumentMarkerList::MarkerType()

We need a method on DocumentMarkerList that returns the marker type the list
supports so we know when we can safely cast a DocumentMarkerList* to an impl
type.

This CL adds a MarkerType param to the constructor of SpellCheckMarkerListImpl
so we can implement this method.

Note that adding this method also requires modifying
GenericDocumentMarkerListImpl to take a MarkerType param (we're going to
eliminate GenericDocumentMarkerListImpl once we add TextMatchMarkerListImpl but
we need this for now).

BUG=707867