Oilpan: Replace most of RefPtrs for Event objects with oilpan's transition types

Source/modules/mediasource/MediaSourceBase.cpp

Index: Source/modules/mediasource/MediaSourceBase.cpp
diff --git a/Source/modules/mediasource/MediaSourceBase.cpp b/Source/modules/mediasource/MediaSourceBase.cpp
index 0f534b047c46f1ef9215d6ab5cd14126afae278f..6662e27945da00bb6fee20e1718f84c58708e326 100644
--- a/Source/modules/mediasource/MediaSourceBase.cpp
+++ b/Source/modules/mediasource/MediaSourceBase.cpp
@@ -346,7 +346,7 @@ void MediaSourceBase::scheduleEvent(const AtomicString& eventName)
 {
     ASSERT(m_asyncEventQueue);
 
-    RefPtr<Event> event = Event::create(eventName);
+    RefPtrWillBeRawPtr<Event> event = Event::create(eventName);
     event->setTarget(this);
 
     m_asyncEventQueue->enqueueEvent(event.release());