MessageEvent constructor should accept a MessagePort as source

Source/core/dom/MessageEvent.cpp

Index: Source/core/dom/MessageEvent.cpp
diff --git a/Source/core/dom/MessageEvent.cpp b/Source/core/dom/MessageEvent.cpp
index bfbc96760418b9b76d5bb6c35e9661e7f014114f..351275ceea98cbbad2501f616afb7d7532f02755 100644
--- a/Source/core/dom/MessageEvent.cpp
+++ b/Source/core/dom/MessageEvent.cpp
@@ -53,7 +53,7 @@ MessageEvent::MessageEvent(const AtomicString& type, const MessageEventInit& ini
     , m_dataType(DataTypeScriptValue)
     , m_origin(initializer.origin)
     , m_lastEventId(initializer.lastEventId)
-    , m_source(initializer.source)
+    , m_source(isValidSource(initializer.source.get()) ? initializer.source : 0)
     , m_ports(adoptPtr(new MessagePortArray(initializer.ports)))
 {
     ScriptWrappable::init(this);