requireSupport_() {
    if (!shaka.offline.Storage.support()) {
      throw new shaka.util.Error(
          shaka.util.Error.Severity.CRITICAL,
          shaka.util.Error.Category.STORAGE,
          shaka.util.Error.Code.STORAGE_NOT_SUPPORTED);
    }
  }
Used by functions that need storage support to ensure that the current
platform has storage support before continuing. This should only be
needed to be used at the start of public methods.

@private