LibWeb: Make BrowsingContext GC-allocated

(And BrowsingContextGroup had to come along for the ride as well.)
This solves a number of nasty reference cycles between browsing
contexts, history items, and their documents.

Userland/Libraries/LibWeb/HTML/Scripting/Environments.cpp

@@ -32,6 +32,7 @@ EnvironmentSettingsObject::~EnvironmentSettingsObject()
 void EnvironmentSettingsObject::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
+    visitor.visit(target_browsing_context);
     for (auto& promise : m_about_to_be_notified_rejected_promises_list)
         visitor.visit(promise);
 }