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This includes:
(1) Adding to BetterTogetherDeviceMetadata Proto.
(2) Adding to Mojo type.
(3) Adding to C++ RemoteDevice and RemoteDeviceRef classes.
(4) Updating tests.

Note that this field is not yet set during enrollment; a follow-up CL
will do this.

Bug: 1106937
Change-Id: I859efa86a7a57cff6eea185e2945a9201d14d508
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318685
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795207}