[Sync] Move SharedModelTypeProcessor to model_impl/.

components/sync/model/model_type_change_processor.h

Index: components/sync/model/model_type_change_processor.h
diff --git a/components/sync/model/model_type_change_processor.h b/components/sync/model/model_type_change_processor.h
index 5eff158cdba1603f01c9af1b6ec4ba9d1a3f9905..7fe49048d81e852943868afbfeba391f88add11e 100644
--- a/components/sync/model/model_type_change_processor.h
+++ b/components/sync/model/model_type_change_processor.h
@@ -8,19 +8,18 @@
 #include <memory>
 #include <string>
 
+#include "components/sync/base/model_type.h"
 #include "components/sync/core/activation_context.h"
 #include "components/sync/model/data_type_error_handler.h"
 #include "components/sync/model/entity_data.h"
 #include "components/sync/model/sync_error_factory.h"
 
 namespace syncer {
-class SyncError;
-}  // namespace syncer
-
-namespace syncer {
 
 class MetadataBatch;
 class MetadataChangeList;
+class ModelTypeService;
+class SyncError;
 
 // Interface used by the ModelTypeService to inform sync of local
 // changes.
@@ -29,6 +28,11 @@ class ModelTypeChangeProcessor : public SyncErrorFactory {
   typedef base::Callback<void(SyncError, std::unique_ptr<ActivationContext>)>
       StartCallback;
 
+  // A factory function to make an implementation of ModelTypeChangeProcessor.
+  static std::unique_ptr<ModelTypeChangeProcessor> Create(
+      ModelType type,
+      ModelTypeService* service);
+
   ModelTypeChangeProcessor();
   ~ModelTypeChangeProcessor() override;