cmd/browser: make compatible with Go 1.8
cmd/browser: make compatible with Go 1.8

The Size field of multipart.FileHeader was added in Go 1.9. We don't
really need it and we might as well remain compatible with Go 1.8.

Change-Id: Ie7ecfa5e23b829af0dbc41ebb95fe079eef94f4d
Reviewed-on: https://upspin-review.googlesource.com/14483
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/exp,upspin/exp