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Remove the |clipping_window| that created as the parent of the
back gesture affordance while dragging from splitview divider. Since
it belongs to the container, which will not be deleted together with
the back gesture affordance. This might make the clipping window above
the splitview divider after SplitViewDivider::SetAlwaysOnTop be called
to stack splitview divider at the bottom of the AlwaysOnTopContainer.
Then the event will not be dispatched correctly to the splitview divider
for resizing.

- Use Layer::SetClipRect(*) to clip the affordance instead.
- Remove useless parameter of CanStartGoingBack(*).
- Reset |dragged_from_splitview_divider_| correctly.

Bug: 1035126
Change-Id: I56af8fcb860f1aa842fe92883f0c122dcc7cedc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1977967
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727193}