Called when a state is received from the front-end.

def set_state(self, sync_data):
        """Called when a state is received from the front-end."""
        # The order of these context managers is important. Properties must
        # be locked when the hold_trait_notification context manager is
        # released and notifications are fired.
        with self._lock_property(**sync_data), self.hold_trait_notifications():
            for name in sync_data:
                if name in self.keys:
                    from_json = self.trait_metadata(name, 'from_json',
                                                    self._trait_from_json)
                    self.set_trait(name, from_json(sync_data[name], self))