WebXR OT samples improvments

1) Add disclaimer on index.html saying WebXR only works for secure
   contexts.
2) Add bold red warning message above the XR button on the published
   samples when the page is accessed in an insecure context.
3) Update python scripts to create a zip file of the source which can
   be downloaded from links in index.html.

Bug: 966565, 973688
Change-Id: I134e886aab173b7f9af87c60af2bb188585c4603
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1665891
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Jacob DeWitt <jacde@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670528}
7 files changed