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The current logic avoids calls to MaybeStartDragWindowFromShelf until
the first shelf drag update, so it can detect the scroll direction
before starting window drag, and then use the scroll value to detect the
initial scroll position. Simpler approach would be to pass the
scroll start gesture's scroll hint in StartGestureDrag.

Additionally, this cl further restricts when window drag can start with
extended hotseat. The window drag will only be allowed if the gesture
started on the shelf (e.g. gesture that starts on hotseat, goes down,
and then up again should not start window drag).

BUG=1043243

Change-Id: I9c70ab4670646e1be27970f37b0e7bcc3f2fc224
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2008618Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733801}