We need to migrate DialRegistry from NetworkChangeNotifier to NetworkConnectionTracker as part of the network service migration (see  https://crbug.com/859133 ). NetworkConnectionTracker currently asserts in its destructor that no observers are registered, which fails because DialRegistry is a leaky singleton so never unregisters itself with the NetworkConnectionTracker.

We can remove the assertion for now, but long term it would be nicer if DialRegistry cleaned up after itself.