Set the opacity directly to element.style.opacity

goog.style.setOpacity handles the IE8 case (but we are doing the same
above in the same function) and Firefox prior to version 3.5 (latest
stable release in April 2011).

See https://developer.mozilla.org/en-US/docs/Web/CSS/opacity
Merge pull request #2714 from fredj/opacity

Set the opacity directly to element.style.opacity