Assume mostly positive values in protobuf encoding

This is a change to the encoding scheme, but Gerrit Code Review
(our only downstream library consumer that matters) only needs to
store positive values.  Positive values encode smaller with the
int32/int64 format than the sint32/sint64 format.

Change-Id: I5bd93592c59b66b8147133ff33f94be51e5490ed
3 files changed