Remove vendor prefixes from isTabVisible() (#2494) (#2505)
Remove vendor prefixes from isTabVisible() (#2494) (#2505)

`document.hidden` is available on supported browsers.

closes #2494 
c3js/c3,c3js/c3,c3js/c3,c3js/c3,masayuki0812/c3,masayuki0812/c3