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This is a reland of 5aa92900

Original change's description:
> Rough in support for multiple selection in holding space.
>
> Holding space needs to support selection of multiple holding space items
> to perform bulk context menu and drag/drop operations on them at a time.
>
> This CL refactors HoldingSpaceItemContextMenu into a shared
> HoldingSpaceItemViewDelegate to facilitate these requirements.
>
> Note that this CL does not change any existing behavior, just lays the
> groundwork for supporting multiple selection.
>
> Bug: 1129981
> Change-Id: If4da6c19ba01bd7104a1b1487350c362d7be1dd4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419136
> Commit-Queue: David Black <dmblack@google.com>
> Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#809625}

Bug: 1129981
Change-Id: I122a335d92a3df5e51933fc2eadf35080d0e87c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425894
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810054}