function setUserAgent(userAgent = undefined) {
  userAgentInternal =
      typeof userAgent === 'string' ? userAgent : getNativeUserAgentString();
}
Override the user agent string with the given value.
This should only be used for testing within the goog.labs.userAgent
namespace.
Pass `null` to use the native browser object instead.
@param {?string=} userAgent The userAgent override.
@return {void}