LibWeb: Add missing visit_edges() for ImageStyleValue

ImageStyleValue has a visit_edges() method, although it is not a
GC-allocated object. This is necessary because it owns a GC-allocated
ImageRequest that we want to visit, instead of using JS::Handle, to
avoid leaks. In the future, we might want to make StyleValue be
GC-allocated.

For now, this change adds missing visit_edges() calls for objects that
own ImageStyleValue.

Userland/Libraries/LibWeb/HTML/HTMLBodyElement.h

@@ -29,6 +29,8 @@ public:
 private:
     HTMLBodyElement(DOM::Document&, DOM::QualifiedName);
 
+    virtual void visit_edges(Visitor&) override;
+
     // ^DOM::Node
     virtual bool is_html_body_element() const override { return true; }