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Per instructions on the Unido design doc [1], this is the first step in
the process of creating a new Unido feature:

1) Add a new SharingMessage to the proto and update metrics mapping
2) Add your new SharingFCMHandler in SharingService constructor
3) Add a new feature in DeviceInfoSpecifics.EnabledFeatures
4) Add your enabled feature to SharingDeviceRegistration
5) Use SharingService to list device candidates, and use the new SendMessageToDevice to send messages

Steps (2-5) will follow in separate CLs, to ease reviewing and ease
other features tracking how to integrate.

[1] https://docs.google.com/document/d/1tvrKCEOaQuZv_j6j61ij_r9sjFlI25x8M8rIRL1DTsA/edit#

Design Doc: https://docs.google.com/document/d/1da8CjO71DlFbBzDcSosFRXTvDsKII_XMCohpZLQ_QbM/edit?userstoinvite=himanshujaju%40google.com&ts=5da89bb1&actionButton=1

Bug: 1015645
Change-Id: If41831a2e71ec88b4771c739be18fd17c18ac6f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867444
Auto-Submit: Sam Goto <goto@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Alex Chau <alexchau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707917}