function isBuffer (obj) {
  return !!obj.constructor && typeof obj.constructor.isBuffer === 'function' && obj.constructor.isBuffer(obj)
}
Dispatches register requests to available U2F tokens. An array of sign
requests identifies already registered tokens.
If the JS API version supported by the extension is unknown, it first sends a
message to the extension to find out the supported API version and then it sends
the register request.
@param {string=} appId
@param {Array<u2f.RegisterRequest>} registerRequests
@param {Array<u2f.RegisteredKey>} registeredKeys
@param {function((u2f.Error|u2f.RegisterResponse))} callback
@param {number=} opt_timeoutSeconds