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A bookmark's GUID must be unique and otherwise BookmarkModel's invariant
can be considered violated.

In this patch, the invariant is enforced via DCHECKs upon node creation,
which is sufficient because GUIDs are inmutable.

Change-Id: Ia3f48e98027ae9a8468bd0dc578843047af79712
Bug: 978430
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031024
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744286}