Skip protobuf-based values for ETW serialization

There is no serialization of protobuf values, and trying to serialize
them DCHECKs so skip them.

R=brucedawson@chromium.org, skyostil@chromium.org

Fixed: 1229719
Change-Id: I42627aee882936068bc2c670a192367d3b945db1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3032622
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#902522}