LibWeb: Add FormattingContext::automatic_content_height()

This function should return the automatic height of the formatting
context's root box.

Until now, we've been relying on some magical handshakes between parent
and child context, when negotiating the height of child context root
boxes. This is a step towards something more reasonable.

Userland/Libraries/LibWeb/Layout/SVGFormattingContext.cpp

@@ -20,6 +20,11 @@ SVGFormattingContext::SVGFormattingContext(LayoutState& state, Box const& box, F
 
 SVGFormattingContext::~SVGFormattingContext() = default;
 
+float SVGFormattingContext::automatic_content_height() const
+{
+    return 0;
+}
+
 void SVGFormattingContext::run(Box const& box, LayoutMode)
 {
     auto& svg_svg_element = verify_cast<SVG::SVGSVGElement>(*box.dom_node());