It seems that once you have a manifest entry, a la:
Then that page (the master entry in the cache) wil
Try to simply delete the manifest file. From the mozzila docs :
Application caches can also become obsolete. If the manifest is removed from the server, the browser removes all application caches that use that manifest, then sends an "obsoleted" event to the application cache object. Then the application cache's status is set to OBSOLETE.
This also worked for me on chrome.