Add certificate field to custom asset section in demo app

[chrisfillmore]

The demo app should include a 'Certificate URL' field in the custom asset section. This would be helpful for testing:

• Fairplay content
• Widevine content where the license server isn't configured to handle certificate requests

I can make a PR for this.

[chrisfillmore]

I'm having a compiler issue. Here is the offending function:

/**
 * @param {!string} uri
 * @return {!Promise}
 * @private
 */
shakaDemo.requestCertificate_ = function(uri) {
  var netEngine = shakaDemo.player_.getNetworkingEngine();
  var requestType = shaka.net.NetworkingEngine.RequestType.APP;
  var uris = /** @type {!Array.<string>} */ ([uri]);
  var request = shaka.net.NetworkingEngine.makeRequest(uris);

  return netEngine.request(requestType, request).then(function(response) {
    return response.data;
  });
};

This is the message I get from the compiler:

[INFO] Compiling the demo app (debug)...
/Users/chrisf/Documents/github_forks/shaka-player/demo/asset_section.js:143: ERROR - Property makeRequest never defined on shaka.net.NetworkingEngine
  var request = shaka.net.NetworkingEngine.makeRequest(uris);
                                           ^^^^^^^^^^^

I thought maybe it was the args I was passing to makeRequest, so I've tried all of the following:
var request = shaka.net.NetworkingEngine.makeRequest([uri]);
var request = shaka.net.NetworkingEngine.makeRequest(uris);
var request = shaka.net.NetworkingEngine.makeRequest(uris, null);
var request = shaka.net.NetworkingEngine.makeRequest(uris, shaka.net.NetworkingEngine.defaultRetryParameters);

I'm not sure what to do. Also, is the type annotation necessary below?

var uris = /** @type {!Array.<string>} */ ([uri]);

Thanks for the help.

[TheModMaker]

It looks like makeRequest isn't exported, so the demo can't access it. You can either add @export to the method or you can create a Request object manually and just not include the fields you don't need. NetworkingEngine will fill any missing fields with defaults.

var request = /** @type {shakaExtern.Request} */ ({uris: [uri]});

That cast is needed since it has missing fields. But the cast for Array.<string> shouldn't be needed.

[chrisfillmore]

Thanks @TheModMaker, I've submitted a PR.