Do not start DnsConfigServiceThread on iOS

DnsConfigService on iOS doesn't watch the config so its result can become
inaccurate at any time.  Disable it to prevent promulgation of inaccurate
DnsConfigs.

https://cs.chromium.org/chromium/src/net/dns/dns_config_service_posix.cc?q=dns_config_service_po&sq=package:chromium&g=0&l=64
shows that this is not implemented on iOS.
Calling the method produces parasite log at startup.

Bug: 852369
Change-Id: I3d24bcb1089fb3c9099b1fffcb829c2a5a2449dc
Reviewed-on: https://chromium-review.googlesource.com/1102317
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567963}

net/base/network_change_notifier_mac.cc

@@ -11,6 +11,7 @@
 #include "base/message_loop/message_loop.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_restrictions.h"
+#include "build/build_config.h"
 #include "net/dns/dns_config_service.h"
 
 namespace net {
@@ -47,13 +48,18 @@ NetworkChangeNotifierMac::NetworkChangeNotifierMac()
       connection_type_(CONNECTION_UNKNOWN),
       connection_type_initialized_(false),
       initial_connection_type_cv_(&connection_type_lock_),
-      forwarder_(this),
-      dns_config_service_thread_(std::make_unique<DnsConfigServiceThread>()) {
+      forwarder_(this) {
   // Must be initialized after the rest of this object, as it may call back into
   // SetInitialConnectionType().
   config_watcher_ = std::make_unique<NetworkConfigWatcherMac>(&forwarder_);
+#if !defined(OS_IOS)
+  // DnsConfigService on iOS doesn't watch the config so its result can become
+  // inaccurate at any time.  Disable it to prevent promulgation of inaccurate
+  // DnsConfigs.
+  dns_config_service_thread_ = std::make_unique<DnsConfigServiceThread>();
   dns_config_service_thread_->StartWithOptions(
       base::Thread::Options(base::MessageLoop::TYPE_IO, 0));
+#endif
 }
 
 NetworkChangeNotifierMac::~NetworkChangeNotifierMac() {