Offline state detection for linux, using new D-Bus library.

Relanding with fix for component build: use NET_EXPORT_PRIVATE
in network_change_notifier_linux.h.

BUG=53473
TEST=see attachment on bug 7469
R=willchan@chromium.org

Review URL: http://codereview.chromium.org/8575013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110374 0039d316-1c4b-4281-b951-d872f2087c98
NetworkChangeNotifier::DisableForTest::~DisableForTest() {
  DCHECK(!g_network_change_notifier);
  g_network_change_notifier = network_change_notifier_;
}

NetworkChangeNotifier::DisableForTest::~DisableForTest() {
  DCHECK(!g_network_change_notifier);
  g_network_change_notifier = network_change_notifier_;
}
