_Util.DOM.getBBox() doesn't choke on Firefox.

Previously, calling getBBox() on a node with the style "display: none"
would throw an error in Firefox (FF bug 612118). The utility method
now returns a safe value in that case, bringing FF's behavior in line
with that of other browsers.
Update comments on getBBox() and test.
Merge pull request #1031 from palantir/FF-bbox-NS_ERROR

_Util.DOM.getBBox() doesn't choke on Firefox.