scrolledwindow: Bug 766569 - Report child natural size unconditionally

scrolledwindow: Bug 766569 - Report child natural size unconditionally

This patch causes the scrolled window default behavior to change in
such a way that the natural size request of the child is unconditionally
reported, which probably should have been the case since day 1.

This should not cause significant fallout since a scrolled window is
normally used to expand/fill, eating up remaining space for scrollable
content - it will however cause the scrolled window to compete for
additional space with siblings, proportionally to the size of the
scrolled window's content.