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The onbeforexrselect event is supposed to be part of GlobalEventHandlers
and separately inherited by {HTML,MathML,SVG}Element, Window, and
Document. The current implementation had added it at the Element
level which isn't correct, causing test failures in the idlharness
test.

Update the implementation to fix the inheritance, which unfortunately
adds a bit of extra boilerplate.

Also add objects to the idlharness test to verify the inheritance.

Spec: https://immersive-web.github.io/dom-overlays/#onbeforexrselect

Fixed: 1108685
Change-Id: I8df73304b7b2d9bd93df30b78953aa9b378f8cad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317813
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846294}