code: async function isValidOrigin(origin, sourceOrigin) {
      // This will fetch the caches from https://cdn.ampproject.org/caches.json the first time it's
      // called. Subsequent calls will receive a cached version.
      const officialCacheList = await caches.list();
      // Calculate the cache specific origin
      const cacheSubdomain = `https://${await createCacheSubdomain(sourceOrigin)}.`;
      // Check all caches listed on ampproject.org
      for (const cache of officialCacheList) {
        const cachedOrigin = cacheSubdomain + cache.cacheDomain;
        if (origin === cachedOrigin) {
          return true;
        }
      }
      return false;
    }
docs: Checks whether the given origin is a valid AMP cache.
  
  @param {String} origin
  @return {boolean} true if origin is valid
  @private