As seen in bug 1745864, many kinds of leaks will cause us to not clean up ContentParents. This in turn means that sContentParents isn't empty when we shut down, which means we hit an assertion. This in effect causes leaks to get turned into a single generic assertion, which hurts distinguishing different leaks.

In bug 1661862, I wrote a little wrapper class that simply cleans up a static linked list when destroyed, rather than asserting. I'll do the same here.

This keeps us from asserting when a ContentParent leaks, in
order to keep different leaks from getting bucketed together
on TreeHerder.

This static variable used to be set in a method called RecvPrivateDocShellsExist,
but it is now always null.

https://hg.mozilla.org/mozilla-central/rev/940d75dffee4
https://hg.mozilla.org/mozilla-central/rev/3b38f7a8b170