script_task.rs: Use self::MixedMessage in handle_msgs
script_task.rs: Use self::MixedMessage in handle_msgs

handle_msgs contains many references to variants of MixedMessage.

The function should directly reference the variants instead of
prepending `MixedMessage::`.

Fixes #9169.

splav/servo,fiji-flo/servo,mbrubeck/servo,dati91/servo,dati91/servo,eddyb/servo,canaltinova/servo,emilio/servo,jimberlage/servo,canaltinova/servo,nnethercote/servo,canaltinova/servo,larsbergstrom/servo,splav/servo,fiji-flo/servo,CJ8664/servo,splav/servo,thiagopnts/servo,paulrouget/servo,canaltinova/servo,upsuper/servo,eddyb/servo,mattnenterprise/servo,notriddle/servo,eddyb/servo,saneyuki/servo,splav/servo,sadmansk/servo,pyfisch/servo,CJ8664/servo,emilio/servo,jimberlage/servo,canaltinova/servo,pyfisch/servo,paulrouget/servo,szeged/servo,saneyuki/servo,fiji-flo/servo,avadacatavra/servo,nrc/servo,anthgur/servo,cbrewster/servo,thiagopnts/servo,sadmansk/servo,canaltinova/servo,larsbergstrom/servo,saneyuki/servo,saneyuki/servo,peterjoel/servo,dsandeephegde/servo,dsandeephegde/servo,emilio/servo,avadacatavra/servo,pyfisch/servo,DominoTree/servo,notriddle/servo,anthgur/servo,SimonSapin/servo,eddyb/servo,nnethercote/servo,larsbergstrom/servo,rnestler/servo,notriddle/servo,DominoTree/servo,paulrouget/servo,notriddle/servo,avadacatavra/servo,notriddle/servo,cbrewster/servo,KiChjang/servo,upsuper/servo,paulrouget/servo,peterjoel/servo,thiagopnts/servo,nnethercote/servo,mbrubeck/servo,upsuper/servo,splav/servo,rnestler/servo,szeged/servo,emilio/servo,dati91/servo,dsandeephegde/servo,ConnorGBrewster/servo,jimberlage/servo,splav/servo,sadmansk/servo,anthgur/servo,splav/servo,larsbergstrom/servo,jimberlage/servo,steveklabnik/servo,avadacatavra/servo,steveklabnik/servo,szeged/servo,pyfisch/servo,ConnorGBrewster/servo,dsandeephegde/servo,sadmansk/servo,emilio/servo,larsbergstrom/servo,avadacatavra/servo,dsandeephegde/servo,dati91/servo,paulrouget/servo,paulrouget/servo,SimonSapin/servo,larsbergstrom/servo,ConnorGBrewster/servo,szeged/servo,rnestler/servo,avadacatavra/servo,peterjoel/servo,cbrewster/servo,nnethercote/servo,cbrewster/servo,mbrubeck/servo,avadacatavra/servo,KiChjang/servo,cbrewster/servo,emilio/servo,anthgur/servo,notriddle/servo,nnethercote/servo,peterjoel/servo,SimonSapin/servo,szeged/servo,saneyuki/servo,larsbergstrom/servo,DominoTree/servo,SimonSapin/servo,KiChjang/servo,canaltinova/servo,emilio/servo,paulrouget/servo,dsandeephegde/servo,pyfisch/servo,peterjoel/servo,dati91/servo,CJ8664/servo,nrc/servo,DominoTree/servo,steveklabnik/servo,larsbergstrom/servo,emilio/servo,upsuper/servo,eddyb/servo,ConnorGBrewster/servo,steveklabnik/servo,nnethercote/servo,SimonSapin/servo,DominoTree/servo,splav/servo,SimonSapin/servo,sadmansk/servo,steveklabnik/servo,mbrubeck/servo,upsuper/servo,fiji-flo/servo,KiChjang/servo,pyfisch/servo,sadmansk/servo,DominoTree/servo,jimberlage/servo,SimonSapin/servo,pyfisch/servo,mattnenterprise/servo,saneyuki/servo,notriddle/servo,emilio/servo,upsuper/servo,peterjoel/servo,splav/servo,avadacatavra/servo,pyfisch/servo,KiChjang/servo,pyfisch/servo,nrc/servo,rnestler/servo,dsandeephegde/servo,mattnenterprise/servo,mattnenterprise/servo,KiChjang/servo,mattnenterprise/servo,sadmansk/servo,nrc/servo,ConnorGBrewster/servo,saneyuki/servo,cbrewster/servo,thiagopnts/servo,DominoTree/servo,jimberlage/servo,rnestler/servo,canaltinova/servo,mbrubeck/servo,CJ8664/servo,emilio/servo,jimberlage/servo,szeged/servo,peterjoel/servo,nnethercote/servo,KiChjang/servo,notriddle/servo,fiji-flo/servo,anthgur/servo,eddyb/servo,nnethercote/servo,ConnorGBrewster/servo,nnethercote/servo,larsbergstrom/servo,notriddle/servo,nrc/servo,dsandeephegde/servo,mbrubeck/servo,peterjoel/servo,nrc/servo,steveklabnik/servo,szeged/servo,cbrewster/servo,szeged/servo,jimberlage/servo,cbrewster/servo,CJ8664/servo,thiagopnts/servo,fiji-flo/servo,upsuper/servo,nnethercote/servo,anthgur/servo,CJ8664/servo,szeged/servo,CJ8664/servo,larsbergstrom/servo,DominoTree/servo,KiChjang/servo,eddyb/servo,splav/servo,nrc/servo,dati91/servo,mbrubeck/servo,rnestler/servo,thiagopnts/servo,sadmansk/servo,anthgur/servo,saneyuki/servo,dati91/servo,KiChjang/servo,dati91/servo,rnestler/servo,mbrubeck/servo,DominoTree/servo,eddyb/servo,ConnorGBrewster/servo,anthgur/servo,paulrouget/servo,ConnorGBrewster/servo,mattnenterprise/servo,notriddle/servo,SimonSapin/servo,paulrouget/servo,peterjoel/servo,paulrouget/servo,peterjoel/servo,fiji-flo/servo,mattnenterprise/servo,steveklabnik/servo,KiChjang/servo,fiji-flo/servo,thiagopnts/servo,DominoTree/servo,rnestler/servo,upsuper/servo,saneyuki/servo,szeged/servo,pyfisch/servo,thiagopnts/servo,mattnenterprise/servo,saneyuki/servo,jimberlage/servo,CJ8664/servo