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This is a reland of d35a5b72, which
includes a fix to prevent the issue of the BookmarkModel being modified
while iterated, which can invalidate iterators due to how GUID
reassignment is implemented.

Original change's description:
> Bookmark sync: use GUID as temporary server ID during commit
>
> This patch effectively relands the logic reverted in
> https://chromium-review.googlesource.com/c/chromium/src/+/1864800 which
> caused crashes in the past (CHECK failures).
>
> In this new form, the underlying issue is addressed: for users that
> don't use GUID-based merging of bookmarks, local GUIDs are reassigned
> randomly to avoid GUID collisions in the local bookmark model.
>
> Bug: 978430,1004205
> Change-Id: I812d1cfe1c12b766799e6c5ec238f95fd16139ad
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1983231
> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#729405}

Bug: 978430, 1004205
Change-Id: Ie86f42a1d8985e325fca5741d43aefc6ef0955c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993295
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730100}