NetworkService: Hook up NetworkChangeNotifier's HistogramWatcher.

Unfortunately, the old patch to set it up in IOThread is still needed,
since the in-process NetworkService is leaked when the OOP network
service is disabled.

Bug=853211

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ic216b2d83d703cff776a77aaea6a2244a9eca367
Reviewed-on: https://chromium-review.googlesource.com/1105068Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568806}

net/base/network_change_notifier.cc

@@ -744,6 +744,7 @@ void NetworkChangeNotifier::NotifyDataReceived(const URLRequest& request,
 void NetworkChangeNotifier::InitHistogramWatcher() {
   if (!g_network_change_notifier)
     return;
+  DCHECK(!g_network_change_notifier->histogram_watcher_);
   g_network_change_notifier->histogram_watcher_.reset(new HistogramWatcher());
   g_network_change_notifier->histogram_watcher_->Init();
 }
@@ -752,6 +753,7 @@ void NetworkChangeNotifier::InitHistogramWatcher() {
 void NetworkChangeNotifier::ShutdownHistogramWatcher() {
   if (!g_network_change_notifier)
     return;
+  DCHECK(g_network_change_notifier->histogram_watcher_);
   g_network_change_notifier->histogram_watcher_.reset();
 }
 

services/network/network_change_manager.cc

@@ -18,9 +18,19 @@ NetworkChangeManager::NetworkChangeManager(
   net::NetworkChangeNotifier::AddNetworkChangeObserver(this);
   connection_type_ =
       mojom::ConnectionType(net::NetworkChangeNotifier::GetConnectionType());
+  // Only initialize the NetworkChangeNotifier's HistogramWatcher if this class
+  // owns the NCN. Otherwise, can't ensure lifetime guarantees are met, so just
+  // rely on the embedder to set it up, if needed.
+  // TODO(mmenke): Once the network service ships, this class should always own
+  // the NCN, and then this can be done unconditionally.
+  if (network_change_notifier_)
+    net::NetworkChangeNotifier::InitHistogramWatcher();
 }
 
 NetworkChangeManager::~NetworkChangeManager() {
+  // Shut down the HistogramWatcher, if it was started in the constructor.
+  if (network_change_notifier_)
+    net::NetworkChangeNotifier::ShutdownHistogramWatcher();
   net::NetworkChangeNotifier::RemoveNetworkChangeObserver(this);
 }