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This CL follows protobuf enum best practices and adds "UNSPECIFIED = 0"
to UserDisplayMode.
https://developers.google.com/protocol-buffers/docs/style#enums

The behavioural change here is that old Chrome clients will interpret
new user display modes (e.g. hypothetically a tabbed window mode) from
the sync server as standalone window mode.
We a more likely to extend the set of window modes rather than browser
tab modes so this is a reasonable fallback.

Change-Id: Ibe989693a6b8fa6b826a0dd8107c5ec71aeb7544
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377671
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802170}