Note that ExceptionCode::kUnknownError is not IDB-specific

third_party/WebKit/Source/core/dom/ExceptionCode.h

 /*
  *  Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
  *  version 2 of the License, or (at your option) any later version.
  *
  *  This library is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 46 matching lines @@
 
   // Others introduced in HTML5:
   kNetworkError,
   kAbortError,
   kURLMismatchError,
   kQuotaExceededError,
   kTimeoutError,
   kInvalidNodeTypeError,
   kDataCloneError,
 
+  // The operation failed for an unknown transient reason (e.g. out of memory).
+  // Note: Rethrowed V8 exception will also have this code.
+  kUnknownError,
+
   // These are IDB-specific.
-  kUnknownError,
   kConstraintError,
   kDataError,
   kTransactionInactiveError,
   kReadOnlyError,
   kVersionError,
 
   // File system
   kNotReadableError,
   kEncodingError,
   kPathExistsError,
@@ skipping 17 matching lines @@
   kV8Error = 1000,
   kV8TypeError,
   kV8RangeError,
   kV8SyntaxError,
   kV8ReferenceError,
 };
 
 }  // namespace blink
 
 #endif  // ExceptionCode_h