DOM overlay input handling

Support WebXR input events while DOM Overlay is active, using a new
"beforexrselect" event to let applications deduplicate input. If that
event is preventDefaulted, i.e. on UI elements, XR input events
are suppressed. If the interaction is in a cross-origin iframe, suppress
XR select events and input updates.

See the draft spec https://immersive-web.github.io/dom-overlays/ for
background information.

Bug: 991747
Change-Id: I3c058454cfcee732a544863bbf6b807437513c87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974820
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731790}
25 files changed