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The field hasn't been populated in a long time so it can be deleted from
the protobuf definition, and the old legacy code (not actually
exercised) deleted.

Change-Id: Iaf65d73e76841c2758679cfd26a93432a438b4ae
Bug: 923287
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2262992
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781803}