Add issuers attribute to the trust token attribute type.

Public doc on Chrome change: https://docs.google.com/document/d/1L7OPuLt9CE5VhT_bGGwp3xej3mehGF-mS1ZZOVX8yp4/edit#

PiperOrigin-RevId: 323121186

Author: Closure Team

externs/browser/html5.js

@@ -3760,9 +3760,11 @@ XMLHttpRequest.prototype.response;
  * Trust Tokens operation (issuance, signing, and redemption) is specified via
  * an instance of the following parameters struct, provided via Fetch, XHR, or
  * the iframe tag.
+ * TODO(b/161890603): Trust Token: Remove the 'issuer' fields after Chrome 86 hits stable.
  * @typedef {{
  *     type: string,
- *     issuer: string,
+ *     issuer: (string|undefined),
+ *     issuers: (!Array<string>|undefined),
  *     refreshPolicy: string,
  *     signRequestData: string,
  *     includeTimestampHeader: boolean,