Add support for the 'pinch-zoom' token to the touch-action CSS property.

proposed spec here: https://rawgit.com/RByers/w3c-pointerevents/touch-action-pinch-zoom/index.html#the-touch-action-css-property

Discussion: https://github.com/w3c/pointerevents/issues/29

I think it's possible that landing this in the spec may get bogged down by politics, but I don't think that should stop us.  IE/Edge have long-shipped this: https://msdn.microsoft.com/en-ca/library/windows/apps/hh767313.aspx.  So we can just ship compatiblity as a "de facto standard".

I think this is urgent because there are scenarios where we'd want people to use passive event listeners (eg. image carousels) where we really don't want to force them into hurting accessibility of their site by disabling pinch-zoom.

See, for example, https://bugs.jqueryui.com/ticket/14993

I think this should be trivial to implement - IIRC all the plumbing is already there due to 'touch-action: manipulation'.  We just have to hook up CSS parser syntax to the WebTouchActionPinchZoom bit.

Ideally we'd also land a web-platform-test for this, but I wouldn't block shipping on it.

dtapuska@ WDYT, worth trying to do in M54?