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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}