Avoid double processing <video> tags in iframes

The call to mw.setConfig('EmbedPlayer.RewriteSelector', '');
which prevents TMH from auto-replacing <video> tags was happening
after scripts were loaded, and thus after the <video> tags were
already processed. On chrome this seemed to just mean doing
double work. On old (3.5) firefox this caused the thing not to
work.

Change-Id: I2de89c14bf2dc3307194137a891caaca5ffb48bf
(cherry picked from commit c469a916dadc82aad5ffd4866a65672d30cc5d18)