[Sync] DeviceInfoService static method and error cleanup.

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 dd9d5f6db2baad3511afe9e1418995bdb2822dd7..8c236ccd70f7678db04b0e540d9846975ceeda2e 100644
--- a/components/sync/model/model_type_sync_bridge.h
+++ b/components/sync/model/model_type_sync_bridge.h
@@ -26,7 +26,11 @@ class MetadataChangeList;
 
 // Interface implemented by model types to receive updates from sync via the
 // SharedModelTypeProcessor. Provides a way for sync to update the data and
-// metadata for entities, as well as the model type state.
+// metadata for entities, as well as the model type state. Sync bridge
+// implementations have the responsibility of providing thier change_processor()
+// with metadata through ModelTypeChangeProcessor::OnMetadataLoaded() as soon as
+// possible. Sync will wait for this method to be called, and afterwards it will
+// start calling into the bridge.
 class ModelTypeSyncBridge : public base::SupportsWeakPtr<ModelTypeSyncBridge> {
  public:
   typedef base::Callback<void(SyncError, std::unique_ptr<DataBatch>)>