NoScript places WebM videos too late behind click-to-play in higher security levels

If setting the security slider to medium-low or a higher level WebM videos start to play but are stopped shortly thereafter by NoScript's click-to-play feature. Expected is that they don't start playing at all until the user has confirmed that. Reported in our blog: https://blog.torproject.org/blog/tor-browser-60-released#comment-183099.

As an example test with: http://ftp.acc.umu.se/pub/debian-meetings/2016/mini-debconf-vienna/webm/Debian_Installer_for_Novena.webm the video starts and a short time later NoScript kicks in.

https://blog.torproject.org/blog/tor-browser-60-released#comment-183106 suggests that this happened in earlier versions, too, although rarely.