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Add depth samples to proposals' index.html, add diagnostic information
(leveraging DOM overlay) to communicate why nothing is rendered (either
pose data or depth data may be missing).

Bug: 1151431
Change-Id: I3ada6936a663ee2cb488a2ff749883a714a07ed7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2578040
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Auto-Submit: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834461}