LibWeb: More specialization of intrinsic sizing layout

This patch adds a separate entry point for this kind of layout.
We override it in BFC to set up initial width/height values for the
BFC root block.

Resulting dimensions are assigned as content_width and content_height
at the end of intrinsic sizing, for each axis, if it's either "auto"
or there's a min-content or max-content constraint in effect.

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h

@@ -22,6 +22,7 @@ public:
     ~BlockFormattingContext();
 
     virtual void run(Box const&, LayoutMode) override;
+    virtual void run_intrinsic_size_determination(Box const&) override;
 
     bool is_initial() const;