It's unused.
The patch also renames nsIExternalSharingAppService.idl as
nsISharingHandlerApp.idl, because the only thing left in the file is the
declaration of nsISharingHandlerApp.
--HG--
rename : uriloader/exthandler/nsIExternalSharingAppService.idl => uriloader/exthandler/nsISharingHandlerApp.idl
extra : rebase_source : c1375d75f3c824c9fadd4164201a7e78c7227020
Nicholas Nethercote