Summary: You must measure children before you lay them out, and YogaLayout didn't do this. This fixes that. We also only recompute the yoga tree if the YogaLayout is laid out with a different size to what it's been measured as. Reviewed By: emilsjolander Differential Revision: D4572237 fbshipit-source-id: 6e86dbf939b06338c1dc2a37b7dafafd548dd390