Oilpan: Fix RawPtr to Member in MediaKeySession::PendingAction

BUG=402766, 403121
NOTRY=true

Review URL: https://codereview.chromium.org/463413002

git-svn-id: svn://svn.chromium.org/blink/trunk@180140 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/modules/encryptedmedia/MediaKeySession.cpp

@@ -105,6 +105,7 @@ public:
     void trace(Visitor* visitor)
     {
         visitor->trace(m_result);
+        visitor->trace(m_event);
     }
 
 private:
@@ -124,7 +125,7 @@ private:
     const Type m_type;
     const Member<ContentDecryptionModuleResult> m_result;
     const RefPtr<ArrayBuffer> m_data;
-    const RefPtrWillBeRawPtr<Event> m_event;
+    const RefPtrWillBeMember<Event> m_event;
 };
 
 // This class allows a MediaKeySession object to be created asynchronously.