Three fields in traffic annotation proto are updated and all fomer annotations
required update.
In proto3, we cannot specify default value for fields. Enum fields get their
first choice by default and boolean fields get false by default. This way if
one does not specify the value for one field, we cannot distinguish it from the
case that the default value is given as input. To overcome this problem, the
semantics::destination field and policy::cookies_allowed are updated.
Also an extra field named comments is added for possible extra required
human readable comments.

Bug: 656607
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: Ib6ac74f2abafb16e18283f1b2559f866f0539eea
Reviewed-on: https://chromium-review.googlesource.com/579188


Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488633}