LibWeb: Add Event.eventPhase constants

Userland/Libraries/LibWeb/DOM/Event.idl

@@ -8,6 +8,10 @@ interface Event {
     readonly attribute EventTarget? currentTarget;
     sequence<EventTarget> composedPath();
 
+    const unsigned short NONE = 0;
+    const unsigned short CAPTURING_PHASE = 1;
+    const unsigned short AT_TARGET = 2;
+    const unsigned short BUBBLING_PHASE = 3;
     readonly attribute unsigned short eventPhase;
 
     undefined stopPropagation();