Add an action for server to send down rollback command.

sync/protocol/sync_protocol_error.h

Index: sync/protocol/sync_protocol_error.h
diff --git a/sync/protocol/sync_protocol_error.h b/sync/protocol/sync_protocol_error.h
index be9232d21989f4dbd6469f7c1c41b002d21a2cb2..aef76501e7c85ed1e86cb8cb27e373e07ec4ce0e 100644
--- a/sync/protocol/sync_protocol_error.h
+++ b/sync/protocol/sync_protocol_error.h
@@ -42,6 +42,9 @@ enum SyncProtocolErrorType {
   // An administrator disabled sync for this domain.
   DISABLED_BY_ADMIN,
 
+  // Client told to stop syncing this device and roll back local data.
+  USER_ROLLBACK,
+
   // The default value.
   UNKNOWN_ERROR
 };
@@ -66,6 +69,9 @@ enum ClientAction {
   // settings page that account is disabled.
   STOP_SYNC_FOR_DISABLED_ACCOUNT,
 
+  // Disable sync and roll back local model to pre-sync state.
+  DISABLE_SYNC_AND_ROLLBACK,
+
   // The default. No action.
   UNKNOWN_ACTION
 };