LibWeb: Make Layout::SVGBox inherit from Layout::Box

This doesn't need anything from Layout::BlockContainer, so let's not
inherit from that.

Userland/Libraries/LibWeb/Layout/SVGBox.cpp

@@ -11,7 +11,7 @@
 namespace Web::Layout {
 
 SVGBox::SVGBox(DOM::Document& document, SVG::SVGElement& element, NonnullRefPtr<CSS::StyleProperties> style)
-    : BlockContainer(document, &element, move(style))
+    : Box(document, &element, move(style))
 {
 }
 

Userland/Libraries/LibWeb/Layout/SVGBox.h

@@ -12,7 +12,7 @@
 
 namespace Web::Layout {
 
-class SVGBox : public BlockContainer {
+class SVGBox : public Box {
 public:
     SVGBox(DOM::Document&, SVG::SVGElement&, NonnullRefPtr<CSS::StyleProperties>);
     virtual ~SVGBox() override = default;