TRUE if this channel’s data is being loaded from the cache. This value
is undefined before the channel fires its OnStartRequest notification
and after the channel fires its OnStopRequest notification.
Get expiration time from cache token. This attribute is equivalent to
nsICachingChannel.cacheToken.expirationTime.
Set/get charset of cache entry. Accessing this attribute is equivalent to
calling nsICachingChannel.cacheToken.getMetaDataElement(“charset”) and
nsICachingChannel.cacheToken.setMetaDataElement(“charset”).