Add missing traces for Supplement<> classes

third_party/WebKit/Source/core/workers/WorkerClients.h

Index: third_party/WebKit/Source/core/workers/WorkerClients.h
diff --git a/third_party/WebKit/Source/core/workers/WorkerClients.h b/third_party/WebKit/Source/core/workers/WorkerClients.h
index 70d5661e60145cbe3b4e36639b559c9b534e48b7..f46c4558a0df67ede9d87479eb0319e96950fbd5 100644
--- a/third_party/WebKit/Source/core/workers/WorkerClients.h
+++ b/third_party/WebKit/Source/core/workers/WorkerClients.h
@@ -31,11 +31,14 @@
 #ifndef WorkerClients_h
 #define WorkerClients_h
 
+#include "core/CoreExport.h"
 #include "platform/Supplementable.h"
 #include "wtf/Forward.h"
 
 namespace blink {
 
+class WorkerClients;
+
 // This is created on the main thread, passed to the worker thread and
 // attached to WorkerGlobalScope when it is created.
 // This class can be used to provide "client" implementations to Workers.
@@ -57,6 +60,8 @@ private:
     WorkerClients() { }
 };
 
+extern template class CORE_EXTERN_TEMPLATE_EXPORT Supplement<WorkerClients>;
+
 } // namespace blink
 
 #endif // WorkerClients_h