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/HTMLObjectElement.cpp

@@ -354,7 +354,7 @@ i32 HTMLObjectElement::default_tab_index_value() const
     return 0;
 }
 
-RefPtr<DecodedImageData const> HTMLObjectElement::image_data() const
+JS::GCPtr<DecodedImageData> HTMLObjectElement::image_data() const
 {
     if (!m_image_request)
         return nullptr;