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Previously we relied solely on a check for the item position to
determine if "Move up" and "Move down" items should be shown in any
given bookmark item menu. This CL updates the check to also makes sure
the bookmark is moveable.

It looks like we had this check at one point and it got lost in a bad
rebase.

BUG=1019178

Change-Id: I595ba7646de23f79bd13e85d0bb2bb1e5d17f413
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887732
Commit-Queue: Theresa  <twellington@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710549}