addVariant(id, func) {
    const variant = new shaka.test.ManifestGenerator.Variant(
        this, /* isPartial= */ false, id);
    if (func) {
      func(variant);
    }
    this.variants.push(variant.build_());
  }
Adds a new variant to the manifest.

@param {number} id
@param {function(!shaka.test.ManifestGenerator.Variant)=} func