Split layout_in_flow_non_replaced_block_level

This function is already quite big and the changes necessary for
properly laying out independent formatting contexts next to floats will
make it even more unwieldy. Split the function in two and add a helper
for calculating the containing block for children and the margin. As
time goes on the independent formatting case is probably going to be
more like the replaced case anyway.

Co-authored-by: Oriol Brufau <obrufau@igalia.com>

Author: mrobinson