LibJS: Switch Agent [[CanBlock]] slot to a enum member

It turns out it was a mistake to make this a virtual since
ServiceWorkerAgents are effectively the exact same as
DedicatedWorkerAgents and SharedWorkerAgents just with [[CanBlock]]
set to false.

Libraries/LibJS/Runtime/Agent.cpp

@@ -22,7 +22,7 @@ bool agent_can_suspend(VM const& vm)
     // NOTE: We default to true if no agent has been provided (standalone LibJS with no embedder).
     if (!agent)
         return true;
-    return agent->can_block();
+    return agent->can_block() == Agent::CanBlock::Yes;
 }
 
 }

Libraries/LibJS/Runtime/Agent.h

@@ -16,12 +16,26 @@ namespace JS {
 // https://tc39.es/ecma262/#sec-agents
 class Agent {
 public:
+    enum class CanBlock {
+        Yes,
+        No,
+    };
+
     virtual ~Agent();
 
-    // [[CanBlock]]
+    CanBlock can_block() const { return m_can_block; }
-    virtual bool can_block() const = 0;
 
     virtual void spin_event_loop_until(GC::Root<GC::Function<bool()>> goal_condition) = 0;
+
+protected:
+    explicit Agent(CanBlock can_block)
+        : m_can_block(can_block)
+    {
+    }
+
+private:
+    // [[CanBlock]]
+    CanBlock m_can_block { false };
 };
 
 bool agent_can_suspend(VM const&);

Libraries/LibWeb/Bindings/MainThreadVM.cpp

@@ -77,7 +77,7 @@ void initialize_main_thread_vm(HTML::EventLoop::Type type)
     VERIFY(!s_main_thread_vm);
 
     s_main_thread_vm = JS::VM::create();
-    s_main_thread_vm->set_agent(make<HTML::SimilarOriginWindowAgent>());
+    s_main_thread_vm->set_agent(HTML::SimilarOriginWindowAgent::create());
 
     auto& agent = as<HTML::Agent>(*s_main_thread_vm->agent());
     agent.event_loop = s_main_thread_vm->heap().allocate<HTML::EventLoop>(type);

Libraries/LibWeb/HTML/Scripting/Agent.h

@@ -21,6 +21,9 @@ struct Agent : public JS::Agent {
     GC::Root<HTML::EventLoop> event_loop;
 
     virtual void spin_event_loop_until(GC::Root<GC::Function<bool()>> goal_condition) override;
+
+protected:
+    using JS::Agent::Agent;
 };
 
 Agent& relevant_agent(JS::Object const&);

Libraries/LibWeb/HTML/Scripting/SimilarOriginWindowAgent.cpp

@@ -11,10 +11,10 @@
 
 namespace Web::HTML {
 
-bool SimilarOriginWindowAgent::can_block() const
+NonnullOwnPtr<SimilarOriginWindowAgent> SimilarOriginWindowAgent::create()
 {
-    // similar-origin window agents can not block, see: https://html.spec.whatwg.org/multipage/webappapis.html#obtain-similar-origin-window-agent
+    // See 'creating an agent' step in: https://html.spec.whatwg.org/multipage/webappapis.html#obtain-similar-origin-window-agent
-    return false;
+    return adopt_own(*new SimilarOriginWindowAgent(CanBlock::No));
 }
 
 // https://html.spec.whatwg.org/multipage/webappapis.html#relevant-agent

Libraries/LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h

@@ -20,6 +20,8 @@ namespace Web::HTML {
 
 // https://html.spec.whatwg.org/multipage/webappapis.html#similar-origin-window-agent
 struct SimilarOriginWindowAgent : public Agent {
+    static NonnullOwnPtr<SimilarOriginWindowAgent> create();
+
     // https://dom.spec.whatwg.org/#mutation-observer-compound-microtask-queued-flag
     // Each similar-origin window agent has a mutation observer microtask queued (a boolean), which is initially false. [HTML]
     bool mutation_observer_microtask_queued { false };
@@ -41,8 +43,11 @@ struct SimilarOriginWindowAgent : public Agent {
     Vector<GC::Root<DOM::Element>>& current_element_queue() { return custom_element_reactions_stack.element_queue_stack.last(); }
     Vector<GC::Root<DOM::Element>> const& current_element_queue() const { return custom_element_reactions_stack.element_queue_stack.last(); }
 
-    // [[CanBlock]]
+private:
-    virtual bool can_block() const override;
+    explicit SimilarOriginWindowAgent(CanBlock can_block)
+        : Agent(can_block)
+    {
+    }
 };
 
 SimilarOriginWindowAgent& relevant_similar_origin_window_agent(JS::Object const&);