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Allow users to reorder their custom links with drag and drop.
Screencast: https://screencast.googleplex.com/cast/NDkxMzY1Mzc2OTgzMDQwMHxmY2JhMTNkYy01NQ

Bug: 851335
Change-Id: I8e77fbdd7c10ec95f60dc5a41e45ae06f7a62543
Reviewed-on: https://chromium-review.googlesource.com/c/1309228
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606189}