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This CLs updates the drag and drop interactions such that only
the DOM is updated as the user drags a tab or group element. The
tab strip model is only changed once the user commits the move by
dropping the element in place.

The DragManager now initializes a new DragSession every time a
drag begins, and uses the session to store and retrieve indices.

This will also make it easier and more consistent to drop a tab or
group from another window into a specific index. The idea is to create
a TabElement or TabGroupElement as a drag enters from another
window and keep most of the code the same.

Bug: 1048894
Change-Id: I92e9866ce2b57aae2736fa81b7a5e9e1b334cdfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065329Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743378}