Have SourceBufferList trace all its object references.

Source/modules/mediasource/SourceBufferList.cpp

Index: Source/modules/mediasource/SourceBufferList.cpp
diff --git a/Source/modules/mediasource/SourceBufferList.cpp b/Source/modules/mediasource/SourceBufferList.cpp
index 3b7383be084a410be7795f307c2def72ecd93752..18b22a585d88325ca3649f11672936ba39c92c64 100644
--- a/Source/modules/mediasource/SourceBufferList.cpp
+++ b/Source/modules/mediasource/SourceBufferList.cpp
@@ -31,6 +31,7 @@
 #include "config.h"
 #include "modules/mediasource/SourceBufferList.h"
 
+#include "core/dom/ExecutionContext.h"
 #include "core/events/GenericEventQueue.h"
 #include "modules/EventModules.h"
 #include "modules/mediasource/SourceBuffer.h"
@@ -99,6 +100,8 @@ ExecutionContext* SourceBufferList::executionContext() const
 
 void SourceBufferList::trace(Visitor* visitor)
 {
+    visitor->trace(m_executionContext);
+    visitor->trace(m_asyncEventQueue);
     visitor->trace(m_list);
     EventTargetWithInlineData::trace(visitor);
 }