Reland "Change the ownership of ash::TabDragDropDelegate"

This is a reland of 1654d660fb0159a5f57d78d9be8a1f37a9c079bf

Same as original; The reverted CL was not the one causing the failure.
Details in: [1].

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=1259127#c2

Original change's description:
> Change the ownership of ash::TabDragDropDelegate
>
> In the current implementation, TabDragDropDelegate runs all its methods
> synchronously and the deletion model is simple: object is deleted
> when it goes out of scope.
> However, to support Lacros' WebUI tab drop, TabDragDropDelegate::Drop()
> becomes naturally asynchronous, since its closure routine needs to
> be called by Lacros.
> Hence, the current ownership model does not work well for lacros,
> since one can not predict when the Drop() closure routine is called.
>
> This CL fixes this by changing the ownership model of
> ash::TabDragDropDelegate to self delete after the Drop()'s closure
> routine is called.
>
> This is a preparation email for fully support WebUI tab drop.
>
> BUG=1236708
> R=oshima@chromium.org
>
> Change-Id: I330f35f9697a27fc3a35454f059cb3377378a718
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3203250
> Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#930161}

Bug: 1236708
Change-Id: I4d09f225c7e9c8d76613ff693ac03152f2c7b27d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3220950
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/main@{#931022}

• ash/drag_drop/drag_drop_controller.cc[diff]
• ash/drag_drop/tab_drag_drop_delegate.cc[diff]
• ash/drag_drop/tab_drag_drop_delegate.h[diff]
• ash/drag_drop/tab_drag_drop_delegate_unittest.cc[diff]