Summary: fillCSSNode was only ever used in RCTShadowView to set the child count which is already performed in insert/remove subview calls and in RCTShadowText is was used to set the measure function which can be done in the initializer instead.
Reviewed By: majak, javache
Differential Revision: D3586418
fbshipit-source-id: de2155daf0f1702c8977bf23183a3b6a650d016b