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/CSS/StyleValues/ImageStyleValue.cpp

@@ -132,7 +132,7 @@ void ImageStyleValue::paint(PaintContext& context, DevicePixelRect const& dest_r
     }
 }
 
-RefPtr<HTML::DecodedImageData const> ImageStyleValue::image_data() const
+JS::GCPtr<HTML::DecodedImageData> ImageStyleValue::image_data() const
 {
     if (!m_image_request)
         return nullptr;