Add deprecation warning to Chrome 76 WebXr samples

This updates the WebXr Samples from 76 (whose Origin Token just expired)
to call out that these samples are deprecated and to redirect visitors
to the official immersive web samples instead.

Also modifies the gitignore file to allow staging of samples without
"git status" or "git cl upload" thinking the repo is dirty.

Fixed: 1022918
Change-Id: I802000baa7fee52b4b24c39feb5e95fda0521439
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906641
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713968}

third_party/webxr_test_pages/.gitignore

-media/
\ No newline at end of file
+media/
+webxr-samples-staging/

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

@@ -87,8 +87,11 @@
       </header>
 
       <main class='main' id='main'>
-        <p>Sample pages demonstrating how to use various aspects of the WebXR API <b><em>as implemented in Chrome versions 76-77.</b></em><br/>
-           Note: these samples will only work when visiting this site securely (https://).</p>
+        <p>
+          <b><em>THESE SAMPLES ARE DEPRECATED - DO NOT USE WITH CHROME 78+</b></em>
+          <br/>
+           For Chrome 78 and later, please use <a href="https://immersive-web.github.io/webxr-samples">these samples</a>, maintained by the Immersive Web Working Group.
+        </p>
 
         <script>
           let pages = [