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The logic is behind a newly-introduced feature toggle: the idea is that
bookmark commits to the server now contain a client tag hash, which is
the hash of the bookmark's GUID.

Populating this field is a simple change in BookmarkLocalChangesBuilder
but additional sophistication is introduced to guard against in-flight
commits and the time a user transitions from the feature being disabled
to being enabled (which can theoretically run into duplicates).

Note that the validation of the client tag hash for incoming updates
is *NOT* behind a feature toggle, but that is believed to be
non-controversial because there aren't many other options for defining
the client tag for bookmarks.

Bug: 1032052
Change-Id: I07448ad405849b29295c3090d3708d3a562e32e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623934Reviewed-by: Rushan Suleymanov <rushans@google.com>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844035}