Oilpan: fix build after r192936.

Source/modules/compositorworker/CompositorWorkerGlobalScope.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CompositorWorkerGlobalScope_h
 #define CompositorWorkerGlobalScope_h
 
 #include "core/dom/FrameRequestCallbackCollection.h"
 #include "core/dom/MessagePort.h"
 #include "core/workers/WorkerGlobalScope.h"
@@ skipping 15 matching lines @@
     void postMessage(ExecutionContext*, PassRefPtr<SerializedScriptValue>, const MessagePortArray*, ExceptionState&);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(message);
 
     int requestAnimationFrame(FrameRequestCallback*);
     void cancelAnimationFrame(int id);
     void executeAnimationFrameCallbacks(double highResTimeNow);
 
     // 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;
 
     FrameRequestCallbackCollection m_callbackCollection;
 };
 
 } // namespace blink
 
 #endif // CompositorWorkerGlobalScope_h