LibWeb: Move layout box rect helpers into FormattingContext

These are only used during layout, and always within formatting context
code, so we might as well put them in FormattingContext and avoid having
to pass the LayoutState around all the time.

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h

@@ -72,6 +72,8 @@ private:
 
     void layout_list_item_marker(ListItemBox const&);
 
+    void measure_scrollable_overflow(Box const&, CSSPixels& bottom_edge, CSSPixels& right_edge) const;
+
     enum class DidIntroduceClearance {
         Yes,
         No,