[Sync] Remove ModelError::IsSet() in favor of base::Optional.

components/sync/model/model_type_sync_bridge.h

Index: components/sync/model/model_type_sync_bridge.h
diff --git a/components/sync/model/model_type_sync_bridge.h b/components/sync/model/model_type_sync_bridge.h
index 693e85b150728e265abf2cdbd9fe8cb9960968d9..74e7df45cda788681eaf2acfd4f6e1338929740b 100644
--- a/components/sync/model/model_type_sync_bridge.h
+++ b/components/sync/model/model_type_sync_bridge.h
@@ -11,6 +11,7 @@
 
 #include "base/callback.h"
 #include "base/memory/weak_ptr.h"
+#include "base/optional.h"
 #include "components/sync/engine/activation_context.h"
 #include "components/sync/model/conflict_resolution.h"
 #include "components/sync/model/entity_change.h"
@@ -62,7 +63,7 @@ class ModelTypeSyncBridge : public base::SupportsWeakPtr<ModelTypeSyncBridge> {
   // combine all change atomically, should save the metadata after the data
   // changes, so that this merge will be re-driven by sync if is not completely
   // saved during the current run.
-  virtual ModelError MergeSyncData(
+  virtual base::Optional<ModelError> MergeSyncData(
       std::unique_ptr<MetadataChangeList> metadata_change_list,
       EntityDataMap entity_data_map) = 0;
 
@@ -71,7 +72,7 @@ class ModelTypeSyncBridge : public base::SupportsWeakPtr<ModelTypeSyncBridge> {
   // |metadata_change_list| in case when some of the data changes are filtered
   // out, or even be empty in case when a commit confirmation is processed and
   // only the metadata needs to persisted.
-  virtual ModelError ApplySyncChanges(
+  virtual base::Optional<ModelError> ApplySyncChanges(
       std::unique_ptr<MetadataChangeList> metadata_change_list,
       EntityChangeList entity_changes) = 0;