Ensure cursor is hidden on scroll
Disable cursor hiding for Chrome versions < 39.0.2171.71

There is a bug for earlier versions of Chrome which leaves
`pointer-events: none` stuck enabled for Google+ after a certain amount
of scrolling.
Tidy cursor hider, move haveChromeVersion to utils.
Merge branch 'mrmr1993-hide-cursor-on-scroll' into post-1.46