[Sync] Add sync_export.h (Step 1 for componentizing sync)

sync/internal_api/public/engine/passive_model_worker.h

Index: sync/internal_api/public/engine/passive_model_worker.h
diff --git a/sync/internal_api/public/engine/passive_model_worker.h b/sync/internal_api/public/engine/passive_model_worker.h
index ec257f63e971c324da8c153e0b283c5631a9c768..4b0160666da17c95de03c2d72dc69afe395ff5ad 100644
--- a/sync/internal_api/public/engine/passive_model_worker.h
+++ b/sync/internal_api/public/engine/passive_model_worker.h
@@ -7,6 +7,7 @@
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "sync/base/sync_export.h"
 #include "sync/internal_api/public/engine/model_safe_worker.h"
 #include "sync/internal_api/public/util/syncer_error.h"
 
@@ -17,7 +18,7 @@ namespace syncer {
 // Implementation of ModelSafeWorker for passive types.  All work is
 // done on the same thread DoWorkAndWaitUntilDone (i.e., the sync
 // thread).
-class PassiveModelWorker : public ModelSafeWorker {
+class SYNC_EXPORT PassiveModelWorker : public ModelSafeWorker {
  public:
   explicit PassiveModelWorker(const MessageLoop* sync_loop);