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Before this patch, server and client protos were not in sync with
respect to the required/optional qualifiers. This lead to obscure
failures when serializing/deserializing messages from and to the
backend.

This patch changes the remaining required fields to optional.

After this patch, we are closer to having wire identical protos.
To completely guarantee this going forward we will introduce more
tooling and sync them on a regular basis.

The fields that changed are all ElementReferenceProtos which
reduce to an empty selector when not set, which is acceptable.

R=rouslan@chromium.org, szermatt@chromium.org

Bug: 806868
Change-Id: If855ad1b882712917e3d883c9f4f607d479fc103
Reviewed-on: https://chromium-review.googlesource.com/1244677Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594634}