When we are beginning to calculate the height, if the vscrollbar_policy
is not GTK_POLICY_NEVER, and there is no min-content-height, then we
need some small non-zero value to get started. The idea is to always
ask for at least enough to fit the horizontal scrollbar.

Simply put, this should be the mirror image of the corresponding width
calculation code.

Those who got used to the buggy behaviour might notice that their
GtkScrolledWindows are not as tall as they used to be.

Fall out from 55196a70

https://bugzilla.gnome.org/show_bug.cgi?id=766530