When generating pprof profiles in proto format, symbolize the profiles.

Change-Id: I2471ed7f919483e5828868306418a63e41aff5c5
Reviewed-on: https://go-review.googlesource.com/34192
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>