Add sample page demonstrating use of mutliple inline session

These session behave identically to the previous sample ("Magic Window")
with the exception that you cannot enter VR, and there are multiple
sessions instead of a single one.

Change-Id: Ic26649a5576cba3643ff903fc0c12fccbfe2afd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835267Reviewed-by: Jacob DeWitt <jacde@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Will Cassella <cassew@google.com>
Cr-Commit-Position: refs/heads/master@{#702626}

third_party/webxr_test_pages/webxr-samples/index.html

@@ -115,6 +115,10 @@
               path: 'magic-window.html',
               description: 'Demonstrates use of a non-exclusive XRSession to present "Magic Window" content.' },
 
+            { title: 'Multiple Magic Window', category: 'Basics',
+              path: 'magic-window-multi.html',
+              description: 'Demonstrates use of multiple non-exclusive XRSessions to present "Magic Window" content.' },
+
             { title: 'Fallback Rendering', category: 'Basics',
               path: 'fallback-rendering.html',
               description: 'Demonstrates a way to fallback to rendering the scene when WebXR isn\'t available..' },