LibWeb: Assign a unique ID to each HTML task

And return that ID when queueing an element task. This is required for
some task tracking - specifically, the HTML ToggleTaskTracker struct.

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -759,10 +759,14 @@ void Element::make_html_uppercased_qualified_name()
 }
 
 // https://html.spec.whatwg.org/multipage/webappapis.html#queue-an-element-task
-void Element::queue_an_element_task(HTML::Task::Source source, JS::SafeFunction<void()> steps)
+int Element::queue_an_element_task(HTML::Task::Source source, JS::SafeFunction<void()> steps)
 {
     auto task = HTML::Task::create(source, &document(), move(steps));
+    auto id = task->id();
+
     HTML::main_thread_event_loop().task_queue().add(move(task));
+
+    return id;
 }
 
 // https://html.spec.whatwg.org/multipage/syntax.html#void-elements

Userland/Libraries/LibWeb/DOM/Element.h

@@ -197,7 +197,7 @@ public:
     void set_custom_properties(Optional<CSS::Selector::PseudoElement>, HashMap<DeprecatedFlyString, CSS::StyleProperty> custom_properties);
     [[nodiscard]] HashMap<DeprecatedFlyString, CSS::StyleProperty> const& custom_properties(Optional<CSS::Selector::PseudoElement>) const;
 
-    void queue_an_element_task(HTML::Task::Source, JS::SafeFunction<void()>);
+    int queue_an_element_task(HTML::Task::Source, JS::SafeFunction<void()>);
 
     bool is_void_element() const;
     bool serializes_as_void() const;

Userland/Libraries/LibWeb/HTML/EventLoop/Task.cpp

@@ -11,15 +11,20 @@
 namespace Web::HTML {
 
 static IDAllocator s_unique_task_source_allocator { static_cast<int>(Task::Source::UniqueTaskSourceStart) };
+static IDAllocator s_task_id_allocator;
 
 Task::Task(Source source, DOM::Document const* document, JS::SafeFunction<void()> steps)
-    : m_source(source)
+    : m_id(s_task_id_allocator.allocate())
+    , m_source(source)
     , m_steps(move(steps))
     , m_document(JS::make_handle(document))
 {
 }
 
-Task::~Task() = default;
+Task::~Task()
+{
+    s_unique_task_source_allocator.deallocate(m_id);
+}
 
 void Task::execute()
 {

Userland/Libraries/LibWeb/HTML/EventLoop/Task.h

@@ -59,6 +59,7 @@ public:
     }
     ~Task();
 
+    int id() const { return m_id; }
     Source source() const { return m_source; }
     void execute();
 
@@ -69,6 +70,7 @@ public:
 private:
     Task(Source, DOM::Document const*, JS::SafeFunction<void()> steps);
 
+    int m_id { 0 };
     Source m_source { Source::Unspecified };
     JS::SafeFunction<void()> m_steps;
     JS::Handle<DOM::Document const> m_document;