LibWeb: Compute position of relative block elements

Section 10.3 "Calculating widths and margins" indicates that the 'left'
and 'right' properties of relatively positioned elements should be set
in accordance with the rules of section 9.4.3.

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h

@@ -47,6 +47,7 @@ public:
 protected:
     void compute_width(Box&);
     void compute_height(Box&);
+    void compute_position(Box&);
 
 private:
     virtual bool is_block_formatting_context() const final { return true; }