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Per instructions on the Unido design doc [1], this is the (2-4) 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

Step (1) is here:

https://chromium-review.googlesource.com/c/chromium/src/+/1867444

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

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

Bug: 1015645
Change-Id: Ia1ca925c107fb9ffe2e148ed0a82997268464657
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867125Reviewed-by: Tim Schumann <tschumann@chromium.org>
Reviewed-by: Alex Chau <alexchau@chromium.org>
Commit-Queue: Sam Goto <goto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707958}