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This change stops duplicate items from being added to the clipboard
history. When an item is copied, if an identical item already exists in
the clipboard history then the existing item will be moved to the front
of the list. (Note: items with some of the same data but copied from a
different location are considered different because they will have a
non-equal DataTransferEndpoint)

Bug: 1131730
Change-Id: I97c8655363ecb34c5ab01eb0323b3e2792ee532b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2578123
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835926}