dashboard: build most recent revision first

dashboard: build most recent revision first

Will fill dashboard down the screen instead of up
when builders get stuck and resume.  Already live.

Also delete dead benchmark code.
I think it is safe to say that if/when we bring
benchmarks back, we will use a different
data model.

Fixes #1228.

R=adg
CC=golang-dev
https://golang.org/cl/4449059