function getFieldValues (header) {
  assert(header !== null)

  const values = []

  for (let value of header.split(',')) {
    value = value.trim()

    if (isValidHeaderName(value)) {
      values.push(value)
    }
  }

  return values
}
@see https://github.com/chromium/chromium/blob/694d20d134cb553d8d89e5500b9148012b1ba299/content/browser/cache_storage/cache_storage_cache.cc#L260-L262
@param {string} header