Summary: There is no reason to malloc a list of 4 child pointers for every CSS node eagerly. Instead, we malloc the list (preserving the default size of 4) when we try to put stuff in it.
Reviewed By: emilsjolander
Differential Revision: D4078012
fbshipit-source-id: 7cdcab03ec4067550a5fee5e1baea14344f3a8f9