Fix trackers for element-under-pointer after node deletion

This CL fixes, behind an experimental flag, how MouseEventManager and
PointerEventManager tracks the element under mouse pointer following
a DOM modification.  The tracker is used for the dispatch of the
boundary events (pointerover, pointerout, pointerenter, pointerleave
and corresponding mouse events).

Previously any DOM modication was ignored, and MEM/PEM would dispatch
to a deleted element which is a no-op further down the event path.
These dropped events caused broken pairing of out/over and
enter/leave events on the JS side.  Re-attaching a deleted
element also caused unexpected pairing.

Related spec discussion:
https://github.com/web-platform-tests/interop/issues/380

Fixed: 1147998, 276329
Change-Id: I8bd4324d4f63a7c1ea2a71fc1b24fd6ea650fc92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4955773
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1223866}