go.tools/cmd/cover: add floating bar with file selector and legend
go.tools/cmd/cover: add floating bar with file selector and legend

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/11076043

tmc/mirror-go.tools,t-yuki/go.tools.old,t-yuki/go.tools.old,tmc/mirror-go.tools,tmc/mirror-go.tools,t-yuki/go.tools.old