Default unspecified value added to enum in proto

Updated the enum in proto to be according to the rules from
https://engdoc.corp.google.com/eng/doc/devguide/proto/index.md?cl=head

BUG=722371

Review-Url: https://codereview.chromium.org/2898683002
Cr-Original-Commit-Position: refs/heads/master@{#473862}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 193a454cb1ee78b5a6cecc6c6e6fb63c1598c40c