Fixing using flex in root node
Fixing using flex in root node

Summary: We should always try to make root node as small as possible, while previously this wasn't functioning this way

Reviewed By: emilsjolander

Differential Revision: D5071164

fbshipit-source-id: b8afef42477d0ed87d0c9fcfd26349e0a0babd6e

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho