WebGPU support for ARCore WebXR

Enables WebGPU sessions with the ARCore backend as long as both the
WebXRIncubations and WebXRLayers features are enabled and SharedBuffers
will be used. WebXRIncubations and SharedBuffers have a flag in
about:flags, but WebXRLayers doesn't, so it's not very trivial to turn
the feature on just yet. I'll try to address that in a follow-up CL.

Bug: 359418629
Change-Id: I1f8b26b8ceca5219c264ea078c1c292f5c63e90e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5939519
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1370315}