Use Date.now() to get animation timestamp

Used to rely on absolute timestamp sent by `requestAnimationFrame` to its callback (specification: http://www.w3.org/TR/animation-timing/#FrameRequestCallback , and the example on the specification page suggests the timestamp is absolute since they substract it to something obtained from `Date.now()`), but on current Chrome versions the timestamp sounds relative.

`Date.now()` is now used whatever comes as parameter.