Delete (rather than just empty) temporary cache after activation

[jeffposnick]

Library Affected:
workbox-precaching

Since Workbox v3, workbox-precaching has been creating a temporary cache that's populated during the install phase, and cleared out during the activate phase. (See #1316.) Following a successful activation, the temporary cache has no entries in it, but the named Cache itself isn't deleted, so it shows up as empty in the cache storage debug tools, and can cause developer confusion.

We might as well delete it following activation, just to prevent that confusion. The cost of recreating the Cache should be negligible.

[philipwalton]

I agree we should delete it, and I also think it should be documented so that when users do encounter it, it's not a surprise.

[jeffposnick]

...actually, no, I just dug into our issue history and found this #1372 which explains how things could go wrong if we did call delete().

I added the delete() to v4 in #1736 but I need to back that out in an upcoming PR, and I'm going to close this issue, as I don't think we can safely delete the cache after all.