The trace tool can generate some interesting profiles, but it was only
exposing them as svg through the web UI.  This adds command line options
to generate the raw pprof file.

Change-Id: I52e4f909fdca6f65c3616add444e3892783640f4
Reviewed-on: https://go-review.googlesource.com/23324Reviewed-by: Russ Cox <rsc@golang.org>