Increase z-index on cannot play file warning

This was causing the text in the warning to be unselectable, and
unclickable. Given that commons has links in that message, this
is a pretty severe usability issue.

See also Ifbe20df984 and bug 60243.

Change-Id: Ibd95d533b91f1f8b6bb9c983486e849c95005e8b
Merge "Increase z-index on cannot play file warning"