Prepare for WebXRLightEstimation OT

Adds UseCounters and the necessary flag integration to conduct an origin
trial for WebXRLightEstimation.

The Origin Trial itself will not be enabled until after the I2E and the
launch bug have been approved.

I2E:
https://groups.google.com/a/chromium.org/g/blink-dev/c/zt3gLmrhTHc/m/9jxodKr5CAAJ

Bug: 1070819
Change-Id: I40f7de94cda4189445bce6996928619fc15f1d15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522123


Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824792}