Don't check connectivity if network is offline.

Some netifs could be ignored by command line flag, --netifs-to-ignore, and
internet could be accessible via those netifs.
Many testing scripts and developers set this for ethernet link and
use ethernet to get testing results.

BUG=internal b/19593970

Review URL: https://codereview.chromium.org/1037693005

Cr-Commit-Position: refs/heads/master@{#322288}

chromecast/net/connectivity_checker.cc

@@ -105,6 +105,11 @@ void ConnectivityChecker::Check() {
   }
   DCHECK(url_request_context_.get());
 
+  // Don't check connectivity if network is offline, because internet could be
+  // accessible via netifs ignored.
+  if (net::NetworkChangeNotifier::IsOffline())
+    return;
+
   // If url_request_ is non-null, there is already a check going on. Don't
   // start another.
   if (url_request_.get())