Run Mutation Observer and Custom Element callbacks consistently at microtask checkpoint

Source/core/dom/Microtask.h

Index: Source/core/dom/Microtask.h
diff --git a/Source/core/loader/UniqueIdentifier.h b/Source/core/dom/Microtask.h
similarity index 89%
copy from Source/core/loader/UniqueIdentifier.h
copy to Source/core/dom/Microtask.h
index c1dc5a2b000338f48b787c9292d80ef1671f5600..4b99bbf487c45930aaa889c1193fd5254a026471 100644
--- a/Source/core/loader/UniqueIdentifier.h
+++ b/Source/core/dom/Microtask.h
@@ -28,14 +28,21 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef UniqueIdentifier_h
-#define UniqueIdentifier_h
+#ifndef Microtask_h
+#define Microtask_h
+
+#include "wtf/Noncopyable.h"
 
 namespace WebCore {
 
-unsigned long createUniqueIdentifier();
+class Microtask {
+public:
+    static void performCheckpoint();
 
-}
+private:
+    explicit Microtask();
+};
 
-#endif // UniqueIdentifier_h
+}
 
+#endif // Microtask_h