Add sample page that leverages prototype Anchor APIs

The phone-ar-plane-detection-anchors.html is mostly a copy-paste of
phone-ar-plane-detection.html with minor changes to leverage anchors
API.

Change-Id: I96a8a02e67dade63030d425dc2c3e58088bf253e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796524
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695783}

third_party/webxr_test_pages/webxr-samples/proposals/index.html

@@ -110,10 +110,11 @@
               path: 'phone-ar-hit-test.html',
               description: 'Demonstrates using the Hit Test API to place virtual objects on real-world surfaces.' },
 
-            { title: 'AR Plane Detection', category: 'Phone AR',
-              path: 'phone-ar-plane-detection.html',
+            { title: 'AR Plane Detection & Anchors', category: 'Phone AR',
+              path: 'phone-ar-plane-detection-anchors.html',
               description: 'Demonstrates using the Plane Detection feature, including implementation of ' +
-                           'synchronous hit test in JavaScript leveraging obtained plane data.' },
+                           'synchronous hit test in JavaScript leveraging obtained plane data and ' +
+                           'anchors API to position objects.' },
           ];
 
           let mainElement = document.getElementById("main");