Prevent scroll position reset on GridLayout hierarchy change (#13386)

State change handling by default clears measured size if the size is set
to undefined; this would cause GridLayout to shrink to zero size because
its child cells have position: absolute. The layout phase recomputes the
size, but in some cases the browser reflows first, affecting the scroll
position of the layout parent. This patch prevents GridLayout from
clearing once-computed sizes during state change.

Change-Id: I832b05f1d774e190f2fffc4128f7b35c61d65711