LibWeb: Improve computation of a layout node's containing block

In particular, we now compute the containing block of boxes with
position:absolute and position:fixed (more) correctly.

Libraries/LibWeb/Layout/LayoutDocument.h

@@ -45,6 +45,8 @@ public:
 
     void did_set_viewport_rect(Badge<Frame>, const Gfx::Rect&);
 
+    virtual bool is_root() const override { return true; }
+
 private:
     LayoutRange m_selection;
 };