Fix a crash when dns prefetching was being shut down without being started....

chrome/browser/net/dns_global.cc

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/net/dns_global.h"
 
 #include <map>
 #include <string>
 
 #include "base/stats_counters.h"
@@ skipping 381 matching lines @@
       EnableDnsPrefetch(enabled);
     }
 
     DLOG(INFO) << "DNS Prefetch service started";
 
     // Start observing real HTTP stack resolutions.
     net::AddDnsResolutionObserver(&dns_resolution_observer);
   }
 }
 
-void ShutdownDnsPrefetch() {
-  DCHECK(NULL != dns_master);
-  dns_master->Shutdown();
+void EnsureDnsPrefetchShutdown() {
+  if (NULL != dns_master)
+    dns_master->Shutdown();
 }
 
 void FreeDnsPrefetchResources() {
   DCHECK(NULL != dns_master);
   delete dns_master;
   dns_master = NULL;
 }
 
 static void DiscardAllPrefetchState() {
   if (!dns_master)
@@ skipping 71 matching lines @@
 }
 
 void TrimSubresourceReferrers() {
   if (NULL == dns_master)
     return;
   dns_master->TrimReferrers();
 }
 
 }  // namespace chrome_browser_net