overview: Simplify code from CL 1560298

Code from this CL is now unnecessarily complicated (and might have been
so all along):
https://chromium-review.googlesource.com/c/chromium/src/+/1560298
There is no need for OverviewSession::GetGridWithOverviewItem() when we
have OverviewItem::overview_grid(). Then there is also no need for
OverviewSession::AddDropTargetForDraggingFromOverview() or
OverviewSession::RemoveDropTargetForDraggingFromOverview(), as the whole
purpose of putting them in OverviewSession was so that they could call
OverviewSession::GetGridWithOverviewItem(). In fact, currently,
OverviewWindowDragController::CompleteNormalDrag() uses
item_->overview_grid()->RemoveDropTarget() whereas
OverviewWindowDragController::ResetGesture() uses
overview_session_->RemoveDropTargetForDraggingFromOverview(item_), and
there is no reason why they should use different code to remove the drop
target.

Test: manual
Change-Id: Ic973e90d08c9b81b00566306e6b3e3e655939ef4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628233
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663084}
3 files changed