[Sync] Convert DTCs to be not RefCounted and NonThreadSafe.

components/sync/api/model_type_change_processor.h

Index: components/sync/api/model_type_change_processor.h
diff --git a/components/sync/api/model_type_change_processor.h b/components/sync/api/model_type_change_processor.h
index 014d45615d6021b0b6f00460459063860cbb46dd..3b7df93ac1a65df31dd46fee57191e78a8388f56 100644
--- a/components/sync/api/model_type_change_processor.h
+++ b/components/sync/api/model_type_change_processor.h
@@ -8,12 +8,12 @@
 #include <memory>
 #include <string>
 
+#include "components/sync/api/data_type_error_handler.h"
 #include "components/sync/api/entity_data.h"
 #include "components/sync/api/sync_error_factory.h"
 #include "components/sync/core/activation_context.h"
 
 namespace syncer {
-class DataTypeErrorHandler;
 class SyncError;
 }  // namespace syncer
 
@@ -56,8 +56,9 @@ class ModelTypeChangeProcessor : public syncer::SyncErrorFactory {
   // inform sync of any unrecoverable errors after calling |callback|, and it is
   // guaranteed to outlive the processor. StartCallback takes a SyncError and an
   // ActivationContext; the context should be nullptr iff the error is set.
-  virtual void OnSyncStarting(syncer::DataTypeErrorHandler* error_handler,
-                              const StartCallback& callback) = 0;
+  virtual void OnSyncStarting(
+      std::unique_ptr<syncer::DataTypeErrorHandler> error_handler,
+      const StartCallback& callback) = 0;
 
   // Indicates that sync is being disabled permanently for this data type. All
   // metadata should be erased from storage.