LibWeb: Make PolicyContainer GC allocated

This is required to store Content Security Policies, as their
Directives are implemented as subclasses with overridden virtual
functions. Thus, they cannot be stored as generic Directive classes, as
it'll lose the ability to call overridden functions when they are
copied.

Libraries/LibWeb/HTML/SessionHistoryEntry.cpp

@@ -18,6 +18,7 @@ void SessionHistoryEntry::visit_edges(Cell::Visitor& visitor)
     Base::visit_edges(visitor);
     visitor.visit(m_document_state);
     visitor.visit(m_original_source_browsing_context);
+    visitor.visit(m_policy_container);
 }
 
 SessionHistoryEntry::SessionHistoryEntry()