LibWeb: Implement getBoundingClientRect() for inline paintables

This fixes the issue that occurred when, after clicking an inline
paintable page would always scroll to the top. The problem was that
`scroll_an_element_into_view()` relies on `get_bounding_client_rect()`
to produce the correct scroll position and for inline paintables we
were always returning zero rect before this change.

Userland/Libraries/LibWeb/Painting/InlinePaintable.h

@@ -22,6 +22,8 @@ public:
     Layout::InlineNode const& layout_node() const;
     auto const& box_model() const { return layout_node().box_model(); }
 
+    CSSPixelRect bounding_rect() const;
+
 private:
     InlinePaintable(Layout::InlineNode const&);