init() {
    // Add the new instance of each mechanism to the muxer.
    const registry = shaka.offline.StorageMuxer.getRegistry_();
    registry.forEach((factory, name) => {
      const mech = factory();
      if (mech) {
        this.mechanisms_.set(name, mech);
      } else {
        shaka.log.info(
            'Skipping ' + name + ' as it is not supported on this platform');
      }
    });

    /** @type {!Array<!Promise>} */
    const initPromises = [];
    for (const mechanism of this.mechanisms_.values()) {
      initPromises.push(mechanism.init());
    }

    return Promise.all(initPromises);
  }
Initialize the storage muxer. This must be called before any other calls.
This will initialize the muxer to use all mechanisms that have been
registered with |StorageMuxer.register|.

@return {!Promise}