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In order to compute the unique position of a newly created
bookmark node, the unique positions of its siblings are required.

This CL persists the unique position of synced bookmark nodes such that
they are available across client restarts.

Bug: 516866
Change-Id: I27c4ac9fbb25488792c6065dfd49069ef126863c
Reviewed-on: https://chromium-review.googlesource.com/1107934
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569987}