Re-land: bindings: Use MessageEventInit for MessageEvent constructor

Source/core/events/MessageEvent.idl

Index: Source/core/events/MessageEvent.idl
diff --git a/Source/core/events/MessageEvent.idl b/Source/core/events/MessageEvent.idl
index eab3089f075451ca57808282efdcd89c86dbf0fd..2dea4ed9fd661aeff1c08bc2cf5f85b3d7d115ac 100644
--- a/Source/core/events/MessageEvent.idl
+++ b/Source/core/events/MessageEvent.idl
@@ -25,16 +25,18 @@
  *
  */
 
+// https://html.spec.whatwg.org/multipage/comms.html#the-messageevent-interfaces
+
 [
-    EventConstructor,
+    Constructor(DOMString type, optional MessageEventInit eventInitDict),
     Exposed=(Window,Worker),
     RaisesException=Constructor,
 ] interface MessageEvent : Event {
-    [InitializedByEventConstructor] readonly attribute DOMString origin;
-    [InitializedByEventConstructor] readonly attribute DOMString lastEventId;
-    [InitializedByEventConstructor] readonly attribute EventTarget? source; // May be a Window or a MessagePort
-    [InitializedByEventConstructor, Custom=Getter] readonly attribute any data;
-    [InitializedByEventConstructor] readonly attribute MessagePort[] ports;
+    readonly attribute DOMString origin;
+    readonly attribute DOMString lastEventId;
+    readonly attribute EventTarget? source; // May be a Window or a MessagePort
+    [Custom=Getter] readonly attribute any data;
+    readonly attribute MessagePort[]? ports;
     [Custom, MeasureAs=InitMessageEvent] void initMessageEvent([Default=Undefined] optional DOMString typeArg,
                                    [Default=Undefined] optional boolean canBubbleArg,
                                    [Default=Undefined] optional boolean cancelableArg,