Make sure we are redirected to the mobile version automatically, by presenting a mobile user-agent string on the /whole/ youtube domain (not just m.youtube.com).
The string is also changed to the simpler ua string used for facebook.com. It gives the same UI, but avoids being seen as an Apple browser, which itself prevents rtsp/3gp content to be served to our browser. We (barely) support html5 video now.
Approved by PS Jenkins bot, Olivier Tilloy.