sw.js: fix TypeError with new workbox.
sw.js: fix TypeError with new workbox.

GerHobbelt/bootstrap,creativewebjp/bootstrap,stanwmusic/bootstrap,gijsbotje/bootstrap,bardiharborow/bootstrap,tagliala/bootstrap,fschumann1211/bootstrap,Hemphill/bootstrap-docs,peterblazejewicz/bootstrap,stanwmusic/bootstrap,inway/bootstrap,peterblazejewicz/bootstrap,kvlsrg/bootstrap,creativewebjp/bootstrap,gijsbotje/bootstrap,Lyricalz/bootstrap,joblocal/bootstrap,joblocal/bootstrap,inway/bootstrap,bootstrapbrasil/bootstrap,seanwu99/bootstrap,fschumann1211/bootstrap,gijsbotje/bootstrap,GerHobbelt/bootstrap,bootstrapbrasil/bootstrap,coliff/bootstrap,Lyricalz/bootstrap,m5o/bootstrap,twbs/bootstrap,nice-fungal/bootstrap,kvlsrg/bootstrap,zalog/bootstrap,nice-fungal/bootstrap,zalog/bootstrap,Hemphill/bootstrap-docs,tjkohli/bootstrap,seanwu99/bootstrap,m5o/bootstrap,tagliala/bootstrap,kvlsrg/bootstrap,nice-fungal/bootstrap,tjkohli/bootstrap,Hemphill/bootstrap-docs,Lyricalz/bootstrap,bardiharborow/bootstrap,yuyokk/bootstrap,fschumann1211/bootstrap,Lyricalz/bootstrap,yuyokk/bootstrap,joblocal/bootstrap,yuyokk/bootstrap,twbs/bootstrap,yuyokk/bootstrap,inway/bootstrap,seanwu99/bootstrap,peterblazejewicz/bootstrap,tagliala/bootstrap,bardiharborow/bootstrap,bootstrapbrasil/bootstrap,bardiharborow/bootstrap,zalog/bootstrap,creativewebjp/bootstrap,gijsbotje/bootstrap,coliff/bootstrap,fschumann1211/bootstrap,GerHobbelt/bootstrap,stanwmusic/bootstrap,joblocal/bootstrap,creativewebjp/bootstrap,seanwu99/bootstrap,stanwmusic/bootstrap,Hemphill/bootstrap-docs