Fixes YouTube on iOS and Android

Allow playback of YouTube videos on iOS and Android devices, where an [issue in the YouTube iFrame player](https://code.google.com/p/gdata-issues/issues/detail?id=5415) means that video playback on those OSes must be started by tapping directly on the iFrame (not via the API).

Restores the work-around for #1214 (which was broken by fb589044bd00318fa4998277ff4808114d1609b4 -- see #1113).

Also, restores YouTube video playback window to full size of MediaElement window; 
reverts 53810f573dd7b16693e5d5a2cfa4375e4542935e (and efab12be14894cb74411aa090d3fc084511c7656).