Summary: The size of the node is already set on the node however there was no way to set the size of the parent to the root so that the root could use percentages. This change fixes this by making the width and height passed to calculate layout be the width and height of the hypothetical parent.
Reviewed By: astreet
Differential Revision: D4611417
fbshipit-source-id: 2fb0eedffa17f0ec89b601722a1717a72e216b9e