Oilpan: Prepare to move EventQueue and its subclasses to Oilpan heap.

Source/modules/encryptedmedia/MediaKeySession.cpp

Index: Source/modules/encryptedmedia/MediaKeySession.cpp
diff --git a/Source/modules/encryptedmedia/MediaKeySession.cpp b/Source/modules/encryptedmedia/MediaKeySession.cpp
index 28101cc1ba78be64fa61952a187fc2b58d656ec4..0bc78cb793a0b1449f5fb572456218ac84acdc2b 100644
--- a/Source/modules/encryptedmedia/MediaKeySession.cpp
+++ b/Source/modules/encryptedmedia/MediaKeySession.cpp
@@ -84,7 +84,12 @@ MediaKeySession::MediaKeySession(ExecutionContext* context, blink::WebContentDec
 MediaKeySession::~MediaKeySession()
 {
     m_session.clear();
+#if !ENABLE(OILPAN)
+    // MediaKeySession and m_asyncEventQueue always become unreachable
+    // together. So MediaKeySession and m_asyncEventQueue are destructed in the
+    // same GC. We don't need to call cancelAllEvents explicitly in Oilpan.
     m_asyncEventQueue->cancelAllEvents();
+#endif
 }
 
 void MediaKeySession::setError(MediaKeyError* error)
@@ -272,6 +277,7 @@ void MediaKeySession::stop()
 
 void MediaKeySession::trace(Visitor* visitor)
 {
+    visitor->trace(m_asyncEventQueue);
     visitor->trace(m_keys);
     EventTargetWithInlineData::trace(visitor);
 }