These two events are the leftovers for support copy/paste/cut/selectall
operations from old text_selection_dialog.js in Gaia. The code for
handling them can be removed since text_selection_dialog.js was removed
in bug 1221462.
The old SelectionStateChangedEvent dispatched by SelectionChanged had
this field, but it has not being ported to CaretStateChangedEvent. We
should include selected text in the event so that Gaia could have a
chance to enhance user experience.