We use:

https://cs.corp.google.com/chromeos_public/src/third_party/chromiumos-overlay/dev-libs/protobuf/protobuf-3.3.0.ebuild?q=protobuf+f:ebuild$&dr=C

Gentoo has:

https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-libs/protobuf/protobuf-3.5.2.ebuild?id=08810387d5ef5493a2bb2ccc4fe50d4e5da4d7d3

Notes:

https://www.chromium.org/chromium-os/gentoo-package-upgrade-process

cros_portage_upgrade \
--upgrade \
--board=x86-generic:daisy:link \
dev-libs/protobuf dev-python/protobuf-python dev-java/protobuf-java 


cros_portage_upgrade \
--board=x86-generic:daisy:link \
--upgrade-deep \
dev-libs/protobuf

./setup_board --board=amd64-generic

cros_portage_upgrade \
--board=amd64-generic:x86-generic:daisy:link \
--upgrade \
dev-libs/protobuf

./build_packages --board=amd64-generic


emerge: there are no ebuilds to satisfy "~dev-libs/protobuf-3.3.0" for /build/amd64-generic/.
(dependency required by "dev-python/protobuf-python-3.3.0::portage-stable" [ebuild])
(dependency required by "chromeos-base/autotest-tests-ltp-0.0.1-r6717::chromiumos" [ebuild])
(dependency required by "chromeos-base/autotest-all-0.0.1-r46::chromiumos" [ebuild])
(dependency required by "chromeos-base/autotest-all" [argument])