Fixes for retain cycles in under ARC

Not sure if you're interested, but I merged your ARC changes over to my master the other day, and in the process of using it tonight I found a bunch of retain cycles that were causing all kinds of leaks. I think I've got everything ironed out, but I haven't tested every possible corner case. Anyway, figured I'd drop you a line and let you know in case you wanted to merge this back into your branch. Cheers!

The commit for this was:
* User/Project@SHA: elklein/chromium-tabs@16c999e8c71134401a78d4d46435517b2271d6ac

I'm a bit new to git/github, so sorry if this is put together a bit awkwardly.