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 <[email protected]>
Reviewed-by: Eric Orth <[email protected]>
Commit-Queue: Paul Jensen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#662879}
diff --git a/net/base/network_change_notifier_fuchsia.cc b/net/base/network_change_notifier_fuchsia.cc
index 2e802ca..f3d4853 100644
--- a/net/base/network_change_notifier_fuchsia.cc
+++ b/net/base/network_change_notifier_fuchsia.cc
@@ -61,6 +61,7 @@
 
 NetworkChangeNotifierFuchsia::~NetworkChangeNotifierFuchsia() {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+  ClearGlobalPointer();
 }
 
 NetworkChangeNotifier::ConnectionType