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Since https://crrev.com/c/1980984 has decoupled latching from scroll
animation state, we can now simply use the latched node for sending of
scrollend and overscroll events to DOM. We simply require that the
latched node be preserved past the GSE so that in the next commit we can
pass it along for dispatching the scrollend event.

This CL removes |scroll_animating_latched_element_id_| (which was
actually unused since the previous CL) and
|scroll_animating_overscroll_target_element_id_|. The latter was
previously needed since ScrollAnimated would clear the latched node if
an animation wasn't created (e.g. at the extent). Since the latch is now
maintained, it can be used in its place.

This CL renames |last_scroller_element_id_| to |last_latched_scroller_|
and makes it match the |CurrentlyScrollingNode|, with the only exception
being that it gets cleared only after the first commit after the
CurrentlyScrollingNode is cleared.

Bug: 940508
Change-Id: I2d0c63bd97c3bc1a9bd0158a08b3352b09bfb1f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981195
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728278}