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Generate unique identifier for each message and store it in a new field
in specifics. This id is used by bridge as storage key and client tag.

Changing tag numbers in the proto is fine because it is not used anywhere
yet.

Bug: 1034932
Change-Id: I5c79fdb70d8897ac300e49e98e78760b8394e52c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015024
Commit-Queue: Rushan Suleymanov <rushans@google.com>
Reviewed-by: Alex Chau <alexchau@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734433}