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The general idea is that, if a client tag is provided upon *creation* of
a bookmark, it should be delivered back to clients during later
GetUpdates.

In order to do so, PersistentBookmarkEntity is extended with a new
member field to accommodate the client tag hash, and the corresponding
conversion to/from protos.

Bug: 1032052
Change-Id: Ie5fe597abb6b913da46a293a5f7bdee4446fcf6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624467Reviewed-by: Rushan Suleymanov <rushans@google.com>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843151}