Packed encoding is the default on the proto3 format. Profiles generated
in the profile.proto format by third parties cannot be decoded by the
Go pprof tool, since its proto decoder does not recognize packed
encoding for repeated fields.

In particular this issue prevents go tool pprof from reading profiles
generated by the version of pprof in github.com/google/pprof

Profiles generated by go tool pprof after this change will use packed
repeating fields, so older versions of pprof will not be able to read
them. pprof will continue to be able to read profiles generated before
this change.

Change-Id: Ife0b353a535ae1e495515b9bcec588dd967e171b
Reviewed-on: https://go-review.googlesource.com/21240Reviewed-by: David Symonds <dsymonds@golang.org>
Run-TryBot: David Symonds <dsymonds@golang.org>