Fix bug causing DCHECKs to be hit in DocumentMarkerController
These DCHECKs check that we're resetting the possibly_has_marker_types_ flag
when markers_ becomes empty. Resetting this flag properly enables a performance
optimization. These DCHECKs are causing sporadic crashes in debug builds; we
believe this is because the markers_ map is holding weak references to its keys
(Node objects), so it's possible for the map to become empty through garbage
collection, which doesn't reset possibly_has_marker_types_.
This CL modifies DocumentMarkerController::PossiblyHasMarkers() to catch this
case. Alternatively, we could handle this case at garbage collection time, but
we believe this approach has better performance characteristics.