Fix DCHECK in network portal detector.

chrome/browser/chromeos/net/network_portal_detector_impl.cc

Index: chrome/browser/chromeos/net/network_portal_detector_impl.cc
diff --git a/chrome/browser/chromeos/net/network_portal_detector_impl.cc b/chrome/browser/chromeos/net/network_portal_detector_impl.cc
index e949d7480ce810e46ce5b7dfc19e656a8f91eede..bb99b868bde721fe7ceca78432789adeb3878cb8 100644
--- a/chrome/browser/chromeos/net/network_portal_detector_impl.cc
+++ b/chrome/browser/chromeos/net/network_portal_detector_impl.cc
@@ -549,7 +549,10 @@ void NetworkPortalDetectorImpl::OnAttemptCompleted(
       same_detection_result_count_ >= kMaxOfflineResultsBeforeReport) {
     OnDetectionCompleted(network, state);
   }
-  ScheduleAttempt(results.retry_after_delta);
+
+  // Observers (via OnDetectionCompleted) may already schedule new attempt.
+  if (is_idle())
+    ScheduleAttempt(results.retry_after_delta);
 }
 
 void NetworkPortalDetectorImpl::Observe(