Fix race in ~NetworkChangeNotifier overloads

Some ~NetworkChangeNotifier overloads post destruction to other
threads, which races ~NetworkChangeNotifier's clearing of
g_network_change_notifier.  Clear g_network_change_notifier at
the begining of ~NetworkChangeNotifier overloads to prevent this.

Bug: 965660
Change-Id: If6d792ea0f1badd7c816308646104e0f72ca5b4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625307
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Commit-Queue: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662879}
8 files changed