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

sync/internal_api/public/base/model_type_payload_map.h

Index: sync/internal_api/public/base/model_type_payload_map.h
diff --git a/sync/internal_api/public/base/model_type_payload_map.h b/sync/internal_api/public/base/model_type_payload_map.h
index 699b84b3529086ddafea9db56c6b621987dfe99c..66eb7f7fd778eb9ce068fa454103cdc81ec27706 100644
--- a/sync/internal_api/public/base/model_type_payload_map.h
+++ b/sync/internal_api/public/base/model_type_payload_map.h
@@ -10,8 +10,12 @@
 #include <map>
 #include <string>
 
+#include "sync/base/sync_export.h"
 #include "sync/internal_api/public/base/model_type.h"
 
+// TODO(akalin): Move the non-exported functions in this file to a
+// private header.
+
 namespace base {
 class DictionaryValue;
 }
@@ -26,7 +30,7 @@ typedef std::map<ModelType, std::string> ModelTypePayloadMap;
 
 // Make a TypePayloadMap from all the types in a ModelTypeSet using a
 // default payload.
-ModelTypePayloadMap ModelTypePayloadMapFromEnumSet(
+SYNC_EXPORT ModelTypePayloadMap ModelTypePayloadMapFromEnumSet(
     ModelTypeSet model_types, const std::string& payload);
 
 ModelTypeSet ModelTypePayloadMapToEnumSet(