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Add Web IDL for XRAnchor and related interfaces along with stub
implementation so that everything compiles & links.

Additionally, refactor XRPlaneSet into an XRSetlike template class to
allow for code reuse between XRPlaneSet, XRAnchorSet, and potentially
other setlikes.

Bug: 992027
Change-Id: I59f1b622dcfb6cd3034ca6f493381443d2094956
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1746955
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688757}