[Sync] Introduce ModelError for USS error handling.

components/reading_list/ios/reading_list_model_unittest.mm

Index: components/reading_list/ios/reading_list_model_unittest.mm
diff --git a/components/reading_list/ios/reading_list_model_unittest.mm b/components/reading_list/ios/reading_list_model_unittest.mm
index 768b34623e8ff0b5ed7ef6037d0712e5232d8104..f5ee9c99cd11e0f223da2c21ef5df436179d3c96 100644
--- a/components/reading_list/ios/reading_list_model_unittest.mm
+++ b/components/reading_list/ios/reading_list_model_unittest.mm
@@ -11,6 +11,7 @@
 #include "components/reading_list/ios/reading_list_model_storage.h"
 #include "components/reading_list/ios/reading_list_store_delegate.h"
 #include "components/sync/model/metadata_change_list.h"
+#include "components/sync/model/model_error.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace {
@@ -109,18 +110,18 @@ class TestReadingListStorage : public ReadingListModelStorage {
     return std::unique_ptr<syncer::MetadataChangeList>();
   }
 
-  syncer::SyncError MergeSyncData(
+  syncer::ModelError MergeSyncData(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityDataMap entity_data_map) override {
     NOTREACHED();
-    return syncer::SyncError();
+    return syncer::ModelError();
   }
 
-  syncer::SyncError ApplySyncChanges(
+  syncer::ModelError ApplySyncChanges(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityChangeList entity_changes) override {
     NOTREACHED();
-    return syncer::SyncError();
+    return syncer::ModelError();
   }
 
   void GetData(StorageKeyList storage_keys, DataCallback callback) override {