Add origin experiments check for amp-list (ampproject#20592)

* Check for origin trial experiments in amp-list

* Fix type check on origin-experiments-impl

* Whitelist origin experiments dependency check

* Refactor load more creation and show/hide logic out of amp-list

* Fix incorrect variable

* Stub getAmpDoc on the AmpDocService prototype

* Short circuit origin trial logic if no amp-list-load-more attribute

build-system/dep-check-config.js

@@ -371,6 +371,8 @@ exports.rules = [
       // Accessing extension-location.calculateExtensionScriptUrl().
       'extensions/amp-script/0.1/amp-script.js->' +
             'src/service/extension-location.js',
+      'extensions/amp-list/0.1/amp-list.js->' +
+            'src/service/origin-experiments-impl.js',
     ],
   },
   {