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.h

@@ -85,6 +85,8 @@ protected:
 private:
     bool set_a_css_declaration(PropertyID, NonnullRefPtr<StyleValue const>, Important);
 
+    virtual void visit_edges(Cell::Visitor&) override;
+
     Vector<StyleProperty> m_properties;
     HashMap<DeprecatedString, StyleProperty> m_custom_properties;
 };