Fix SingleFlex Child condition
Fix SingleFlex Child condition

Summary:
Fixes the improper `singleFlexChild` optimization. In the case when all the childs have `flex-grow:0 flex-grow:0` except one child with `flex-grow:1 flex-shrink:1`, then one can simply measure all the non-flexing children and then give the flexing child all the remaining space.

Also added a test case which reproduced the bug

Reviewed By: IanChilds

Differential Revision: D8782684

fbshipit-source-id: ffd4d35b6122f82111b987540efb23bd2a8da5a2

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