Bug1100528-[Gallery[RTL]-Mirror slider thumb to progress left while playing video in gallery r=djf

nit: I find conditional expressions hard to read, especially when they are broken across multiple lines. It is just as efficient, but more readable to do:

var startEdge = navigator.mozL10n.language.direction === 'ltr ? 'left' : 'right';
playHead.style[startEdge] = percent;