LibWeb: Make HTML::DecodedImageData to be GC-allocated

This change fixes GC-leak caused by following mutual dependency:
- SVGDecodedImageData owns JS::Handle for Page.
- SVGDecodedImageData is owned by visited objects.
by making everything inherited from HTML::DecodedImageData and
ListOfAvailableImages to be GC-allocated.

Generally, if visited object has a handle, very likely we leak
everything visited from object in a handle.

Userland/Libraries/LibWeb/HTML/DecodedImageData.h

@@ -14,7 +14,7 @@
 namespace Web::HTML {
 
 // https://html.spec.whatwg.org/multipage/images.html#img-req-data
-class DecodedImageData : public RefCounted<DecodedImageData> {
+class DecodedImageData : public JS::Cell {
 public:
     virtual ~DecodedImageData();