WebIDL tests assert error messages are "", if they are defined on the prototype

exceptions.html:77 has:

if (exception.hasOwnProperty("message")) {
  ...
} else {
  // Error.prototype.message
  assert_equals(exception.message, "");
}

Blink is failing that assertion, but I think the assertion may be wrong or out of date. Could we change the assertion or add a link to the relevant spec, and I will change Blink? I can't find anything that specifies what this text should be nor that it should be an empty string.

Start at WebIDL/ecmascript-binding/es-exceptions/exceptions.html:77 and inspect the existing prototype-message assertion. Check the relevant WebIDL or ECMAScript specification text to determine whether an empty message is required; then update the assertion or add the specification link as appropriate. Run the affected WebIDL exceptions test to verify the result.