Oilpan: fix build after r192936.

Source/modules/compositorworker/CompositorWorkerGlobalScope.cpp

Index: Source/modules/compositorworker/CompositorWorkerGlobalScope.cpp
diff --git a/Source/modules/compositorworker/CompositorWorkerGlobalScope.cpp b/Source/modules/compositorworker/CompositorWorkerGlobalScope.cpp
index 21d4b7d07ed5a334835a68bc499165cb1642197e..b01cf90d47c9e8421438725f30a2a9570ed3fd80 100644
--- a/Source/modules/compositorworker/CompositorWorkerGlobalScope.cpp
+++ b/Source/modules/compositorworker/CompositorWorkerGlobalScope.cpp
@@ -32,6 +32,14 @@ CompositorWorkerGlobalScope::~CompositorWorkerGlobalScope()
 {
 }
 
+DEFINE_TRACE(CompositorWorkerGlobalScope)
+{
+#if ENABLE(OILPAN)
+    visitor->trace(m_callbackCollection);
+#endif
+    WorkerGlobalScope::trace(visitor);
+}
+
 const AtomicString& CompositorWorkerGlobalScope::interfaceName() const
 {
     return EventTargetNames::CompositorWorkerGlobalScope;