checkPlayableVariants_(manifest) {
    const valid = manifest.variants.some(shaka.util.StreamUtils.isPlayable);

    // If none of the variants are playable, throw
    // CONTENT_UNSUPPORTED_BY_BROWSER.
    if (!valid) {
      throw new shaka.util.Error(
          shaka.util.Error.Severity.CRITICAL,
          shaka.util.Error.Category.MANIFEST,
          shaka.util.Error.Code.CONTENT_UNSUPPORTED_BY_BROWSER);
    }
  }
Confirm some variants are playable. Otherwise, throw an exception.
@param {!shaka.extern.Manifest} manifest
@private