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.h

@@ -81,7 +81,7 @@ private:
 
     Representation m_representation { Representation::Unknown };
 
-    RefPtr<DecodedImageData const> image_data() const;
+    JS::GCPtr<DecodedImageData> image_data() const;
 
     JS::GCPtr<SharedImageRequest> m_image_request;
 };