Lazy create children list when first child is added
Lazy create children list when first child is added

Summary:
@public

We don't need to allocate a list for every node since leaf nodes don't have children.

Reviewed By: emilsjolander

Differential Revision: D4130818

fbshipit-source-id: 80d3e98fce9d2daa0676fd1cbed0e81edcf7adb3

facebook/yoga,rmarinho/yoga,yihuang/css-layout,yihuang/css-layout,rmarinho/yoga,yihuang/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,yihuang/css-layout,facebook/yoga,rmarinho/yoga,rmarinho/yoga,yihuang/css-layout,rmarinho/yoga,rmarinho/yoga,facebook/yoga,yihuang/css-layout,facebook/css-layout,facebook/yoga,yihuang/css-layout,yihuang/css-layout,rmarinho/yoga,facebook/yoga,facebook/css-layout,facebook/css-layout,yihuang/css-layout,facebook/css-layout,rmarinho/yoga,yihuang/css-layout,rmarinho/yoga,rmarinho/yoga,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga