Oilpan: switch some leftover raw references over to traced Member<>s.

Source/modules/mediastream/RTCDataChannel.cpp

Index: Source/modules/mediastream/RTCDataChannel.cpp
diff --git a/Source/modules/mediastream/RTCDataChannel.cpp b/Source/modules/mediastream/RTCDataChannel.cpp
index 45e3384c37446d5830b24c8925b364bfbfdc9ac5..2fb38e7f52c9e8bdcdd43a7441f2c518b0ba7655 100644
--- a/Source/modules/mediastream/RTCDataChannel.cpp
+++ b/Source/modules/mediastream/RTCDataChannel.cpp
@@ -334,6 +334,7 @@ void RTCDataChannel::clearWeakMembers(Visitor* visitor)
 
 DEFINE_TRACE(RTCDataChannel)
 {
+    visitor->trace(m_executionContext);
     visitor->trace(m_scheduledEvents);
     visitor->template registerWeakMembers<RTCDataChannel, &RTCDataChannel::clearWeakMembers>(this);
     RefCountedGarbageCollectedEventTargetWithInlineData<RTCDataChannel>::trace(visitor);