LibWeb: Support unique task sources

Some elements, like HTMLMediaElement, must have a unique task sources
for every instance of that element that is created. Support this with a
simple wrapper around IDAllocator.

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

@@ -15,6 +15,8 @@
 
 namespace Web::HTML {
 
+struct UniqueTaskSource;
+
 class Task {
 public:
     // https://html.spec.whatwg.org/multipage/webappapis.html#generic-task-sources
@@ -29,6 +31,10 @@ public:
         Microtask,
         TimerTask,
         JavaScriptEngine,
+
+        // Some elements, such as the HTMLMediaElement, must have a unique task source per instance.
+        // Keep this field last, to serve as the base value of all unique task sources.
+        UniqueTaskSourceStart,
     };
 
     static NonnullOwnPtr<Task> create(Source source, DOM::Document const* document, JS::SafeFunction<void()> steps)
@@ -52,4 +58,11 @@ private:
     JS::Handle<DOM::Document const> m_document;
 };
 
+struct UniqueTaskSource {
+    UniqueTaskSource();
+    ~UniqueTaskSource();
+
+    Task::Source const source;
+};
+
 }