WebXR depth: code change required for OT integration

Add origin trial feature name for WebXR Depth Sensing API, as per
instructions found in:
https://chromium.googlesource.com/chromium/src/+/master/docs/origin_trials_integration.md

This prepares the feature for an OT. The OT will not be enabled until we
get the necessary sign-offs from web platform owners and security +
privacy reviewers (see launch bug below).

Bug: 1131625
Change-Id: Ib139254efc5bab3bfe6578e6c929e2a26ee7aca2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441490Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812796}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -2124,6 +2124,7 @@
     },
     {
       name: "WebXRDepth",
+      origin_trial_feature_name: "WebXRDepth",
       depends_on: ["WebXRARModule"],
       status: "experimental",
     },