Summary: Adds two methods ```AddChild``` and ```RemoveChild``` to be able to add/remove children without the need to know there position. Closes https://github.com/facebook/yoga/pull/513 Reviewed By: emilsjolander Differential Revision: D4914242 Pulled By: splhack fbshipit-source-id: df187ba71a1bae0b3b0649624f3c29401d5f8804