SVGElement.offsetParent is deprecated

[drosen0]

In Chrome 48, when using the tooltip() visualization, this warning is logged to the console:

'SVGElement.offsetParent' is deprecated and will be removed in M50, around April 2016. See https://www.chromestatus.com/features/5724912467574784 for more details.

In contour-utils.js: _.nw.getCentroid(), an assumption is made in the case where offsetParent is falsy, with a comment explaining that Firefox doesn't support offsetParent. It's also referenced in _.nw.getStyle()

Now that it's no longer supported in Chrome either, we need to ensure that the assumption is still valid and remove or replace the code referencing offsetParent.

[drosen0]

Chrome 50 beta has been released. At first glance, normal tooltips do not appear have any rendering issues. I have not yet done thorough testing.