Oilpan: fix build after r192936.

Source/modules/compositorworker/CompositorWorkerGlobalScope.h

Index: Source/modules/compositorworker/CompositorWorkerGlobalScope.h
diff --git a/Source/modules/compositorworker/CompositorWorkerGlobalScope.h b/Source/modules/compositorworker/CompositorWorkerGlobalScope.h
index c573cadaba6032c387cbb8544674f56b13461b18..7b6be9a1881232f1eb076917365c36351b2e75d9 100644
--- a/Source/modules/compositorworker/CompositorWorkerGlobalScope.h
+++ b/Source/modules/compositorworker/CompositorWorkerGlobalScope.h
@@ -33,6 +33,8 @@ public:
     // ExecutionContext:
     bool isCompositorWorkerGlobalScope() const override { return true; }
 
+    DECLARE_VIRTUAL_TRACE();
+
 private:
     CompositorWorkerGlobalScope(const KURL&, const String& userAgent, CompositorWorkerThread*, double timeOrigin, const SecurityOrigin*, PassOwnPtrWillBeRawPtr<WorkerClients>);
     CompositorWorkerThread* thread() const;