Fix video preview not working on Safari Mobile (iPad)

Video previewer is not working on Safari Mobile (iPad).

When displaying the video preview on the iPad 2 errors are thrown in the console:
https://github.com/Polymer/polymer/blob/master/lib/utils/async.html#L33
https://github.com/Polymer/polymer/blob/master/lib/utils/flush.html#L35

The errors are related to this:
https://github.com/nuxeo/nuxeo-ui-elements/commit/d15c70d7859a4c0902c932832a58ab2b0de12921#diff-b108707615ef2f8ffbb6a0010c0f718cR219
Commenting this line will "fix" the error messages, but then the properties are not propagated.

It seems related with videojs and if we set `this.movingMediaElementInDOM` to `true` (https://github.com/videojs/video.js/blob/5.x/src/js/tech/html5.js#L260) it's possible to play the video on iPad.

This issue was identified and described here: https://jira.nuxeo.com/browse/NXP-24484?focusedCommentId=331749&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-331749