LibWeb: Make tiled backgrounds scroll with content

Previously the page background was always draw relative to the viewport
instead of following with the content. This should eventually become
an opt-in mode (via CSS "background-attachment") but for now let's have
the default behavior be that backgrounds scroll with content.

Also take this opportunity to move the background painting code from
the two web views to a shared location in InitialContainingBlockBox.

Userland/Libraries/LibWeb/Layout/InitialContainingBlockBox.h

@@ -41,6 +41,8 @@ public:
     void paint_all_phases(PaintContext&);
     virtual void paint(PaintContext&, PaintPhase) override;
 
+    void paint_document_background(PaintContext&);
+
     virtual HitTestResult hit_test(const Gfx::IntPoint&, HitTestType) const override;
 
     const LayoutRange& selection() const { return m_selection; }