Reserve Cache name prefix for internal browser use

[wanderview]

For various reasons the browser may wish to store data in the CacheStorage facility for the origin. For example, to implement ServiceWorkerGlobalScope.scriptCache.

It would be nice to reserve a cache name prefix for use for by the browser for these purposes. Something like 'internal'. So if content tries to pass something like '__internal__whatever' to CacheStorage, then the promises will always reject.

[wanderview]

Just wanted to note that I've implemented things on our end such that we don't need this. I'm going to close the issue unless someone else feels it would be useful for other implementations.