Use offsetX and offsetY if available

Use the mouse event `offsetX` and `offsetY` properties if available instead of computing the value in javascript. See http://www.w3.org/TR/cssom-view/#dom-mouseevent-pagex

Calling `goog.style.getRelativePosition` will cause a reflow (because `getBoundingClientRect` is called in this function)

relative `to` the viewport.