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This is required to make it possible to use ToplevelWindowEventHandler
for handling window dragging in Exo's extended-drag sessions, e.g:
LaCrOS Chrome's tab dragging implementation.

Extended drag sessions are driven by ash::DragDropController, which sets
capture to its internal DragDropTracker instance. This changes
ToplevelWindowEventHandler so that it does not assumes capture must be
set on the dragged window when its AttemptToStartDrag() functions are
called.

This CL does not imply in any functional change. It will start to be
used in crrev.com/c/2401280 followup change.

R=oshima@chromium.org

Bug: 1099418
Change-Id: I8d76f72675eb123fbd41093efa1d9643adeb482c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2483662Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#821599}