Oilpan: fix build after r179903.

Source/modules/serviceworkers/ServiceWorkerRegistration.cpp

Index: Source/modules/serviceworkers/ServiceWorkerRegistration.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorkerRegistration.cpp b/Source/modules/serviceworkers/ServiceWorkerRegistration.cpp
index a1931de787191456eefd1d3fa9804697c8f95441..ed64100f932e181505add1178506e4b8efb8f8f9 100644
--- a/Source/modules/serviceworkers/ServiceWorkerRegistration.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerRegistration.cpp
@@ -145,4 +145,12 @@ ServiceWorkerRegistration::ServiceWorkerRegistration(ExecutionContext* execution
     m_outerRegistration->setProxy(this);
 }
 
+void ServiceWorkerRegistration::trace(Visitor* visitor)
+{
+    visitor->trace(m_installing);
+    visitor->trace(m_waiting);
+    visitor->trace(m_active);
+    EventTargetWithInlineData::trace(visitor);
+}
+
 } // namespace blink