login: Ignore trailing zeroes in install attributes protobuf.

BUG=chromium:389481
TEST=updated unit test
TEST=passed 'cros_workon_make --test'
Change-Id: I410a4721128f75b74e9f25df69856eac0f309185
Previous-Reviewed-on: https://chromium-review.googlesource.com/209921
(cherry picked from commit fbf8933e2b8ca5721c6a12796a9743b72ec92452)
Reviewed-on: https://chromium-review.googlesource.com/209922
Reviewed-by: Thiemo Nagel <tnagel@chromium.org>
Commit-Queue: Thiemo Nagel <tnagel@chromium.org>
Tested-by: Thiemo Nagel <tnagel@chromium.org>
3 files changed