LibWeb: Fix memory leak in CSS::ImageStyleValue

Before this change, whenever ImageStyleValue had a non-null
`m_image_request`, it was always leaked along with everything related
to the document to which this value belongs. The issue arises due to
the use of `JS::Handle` for the image request, as it introduces a
cyclic dependency where `ImageRequest` prevents the `CSSStyleSheet`,
that owns `ImageStyleValue`, from being deallocated:
- ImageRequest
- FetchController
- FetchParams
- Window
- HTMLDocument
- HTMLHtmlElement
- HTMLBodyElement
- Text
- HTMLHeadElement
- Text
- HTMLMetaElement
- Text
- HTMLTitleElement
- Text
- HTMLStyleElement
- CSSStyleSheet

This change solves this by visiting `m_image_request` from
`visit_edges` instead of introducing new heap root by using
`JS::Handle`.

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.cpp

@@ -9,6 +9,7 @@
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/CSSStyleDeclaration.h>
 #include <LibWeb/CSS/Parser/Parser.h>
+#include <LibWeb/CSS/StyleValues/ImageStyleValue.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/Infra/Strings.h>
@@ -38,6 +39,15 @@ PropertyOwningCSSStyleDeclaration::PropertyOwningCSSStyleDeclaration(JS::Realm&
 {
 }
 
+void PropertyOwningCSSStyleDeclaration::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    for (auto& property : m_properties) {
+        if (property.value->is_image())
+            property.value->as_image().visit_edges(visitor);
+    }
+}
+
 String PropertyOwningCSSStyleDeclaration::item(size_t index) const
 {
     if (index >= m_properties.size())