update_payload: Regenerate protobuf with protoc 2.5

Goobuntu trusty uses protobuf 2.5.0 python modules, but the chroot uses
version 2.6.1. Protobufs compiled with protoc 2.6.1 (inside the chroot)
are not compatible with the python modules installed in trusty, so they
can only run inside the chroot. Nevertheless, protobufs compiled with
protoc 2.5.0 work with the new library version.

BUG=chromium:531884
TEST=`./paycheck.py` inside and outside the chroot with libprotobuf 2.5.0

Change-Id: Ifeaf203a9c891156dd487249673a8264c74fd7e3
Reviewed-on: https://chromium-review.googlesource.com/299814


Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>