dashboard/coordinator: clean up trailer handling
dashboard/coordinator: clean up trailer handling

This got lost before in my git stash stack.

Change-Id: Id73cdb89e73ab83c26971c6c1c4e0fc74d91018f
Reviewed-on: https://go-review.googlesource.com/2521
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

cw2018/tools,sshetty10/tools,dmitris/tools,cmars/tools,cw2018/tools,4ad/tools,glasser/golang-tools,springning/tools,dmitris/tools,matryer/tools,jdhenke/tools,runcom/tools,sshetty10/tools,nicot/tools,josharian/tools,4ad/tools,dmitris/tools,glasser/golang-tools,syreclabs/go-tools,argandas/tools,glasser/golang-tools,runcom/tools,magiccao/tools,argandas/tools,cmars/tools,cmars/tools,4ad/tools,cmars/tools,golang/tools,syreclabs/go-tools,maddyonline/tools,syreclabs/go-tools,matryer/tools,golang/tools,josharian/tools,syreclabs/go-tools,sshetty10/tools,maddyonline/tools,magiccao/tools,magiccao/tools,matryer/tools,argandas/tools,josharian/tools,nicot/tools,springning/tools,runcom/tools,syreclabs/go-tools,nicot/tools,golang/tools,jdhenke/tools,matryer/tools,syreclabs/go-tools,magiccao/tools,dmitris/tools,glasser/golang-tools,runcom/tools,jdhenke/tools,jdhenke/tools,golang/tools,argandas/tools,sshetty10/tools,springning/tools,cw2018/tools,springning/tools,maddyonline/tools,maddyonline/tools,golang/tools,josharian/tools,nicot/tools,cw2018/tools