The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8e20a770c6072f9563c9c12f001876c7ae59469a

commit 8e20a770c6072f9563c9c12f001876c7ae59469a
Author: Mounir Lamouri <mlamouri@chromium.org>
Date: Wed Jul 11 19:54:58 2018

Autoplay: allow autoplay for kiosk applications.

The different with other Chrome Apps is that <webview> should also be
allowed to autoplay if they are inside a Kiosk application.

Bug:  856659 
Change-Id: If51ca43549d3829eeb9fe25309770467c1fc14c2
Reviewed-on: https://chromium-review.googlesource.com/1115096
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574292}
[modify] https://crrev.com/8e20a770c6072f9563c9c12f001876c7ae59469a/chrome/browser/extensions/chrome_extension_web_contents_observer.cc
[modify] https://crrev.com/8e20a770c6072f9563c9c12f001876c7ae59469a/chrome/test/base/ash_test_environment_chrome.cc
[modify] https://crrev.com/8e20a770c6072f9563c9c12f001876c7ae59469a/chrome/test/base/ash_test_environment_chrome.h

I'm still observing a webview within an autoolaunched kiosk app not being allowed to autoplay on Chrome OS 70. Source code seems to indicate this should work, but alas, still stuck. Any suggestions?