cmd/cover, cmd/trace, and cmd/pprof all open browsers.
'go bug' will soon also open a browser.
It is time to unify the browser-handling code.

Change-Id: Iee6b443e21e938aeaaac366a1aefb1afbc7d9b2c
Reviewed-on: https://go-review.googlesource.com/29160
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>