Explicitly reserve space for layout paddings to avoid slot growing.

Explicitly reserve space for layout paddings to avoid slot growing.

Without this feature, CssLayout could cause the slot to grow to over
then parent's padding, which in the next layout phase would cause the
CssLayout to be measured as a little bigger, causing a parent layout
with undefined size to grow to make room for the bigger child,
which again would make the CssLayout grow...