Make the SystemMonitor observer list thread safe....

base/observer_list_threadsafe.h

 // 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.
 
 #ifndef BASE_OBSERVER_LIST_THREADSAFE_H_
 #define BASE_OBSERVER_LIST_THREADSAFE_H_
 
 #include <vector>
 #include <algorithm>
 
@@ skipping 63 matching lines @@
   }
 
   // Remove an observer from the list.
   // If there are pending notifications in-transit to the observer, they will
   // be aborted.
   // RemoveObserver MUST be called from the same thread which called
   // AddObserver.
   void RemoveObserver(ObserverType* obs) {
     ObserverList<ObserverType>* list = NULL;
     MessageLoop* loop = MessageLoop::current();
+    if (!loop)
+      return;  // On shutdown, it is possible that current() is already null.
     {
       AutoLock lock(list_lock_);
-      DCHECK(observer_lists_.find(loop) != observer_lists_.end()) <<
-          "RemoveObserver called on for unknown thread";
       list = observer_lists_[loop];
+      if (!list) {
+        NOTREACHED() << "RemoveObserver called on for unknown thread";
+        return;
+      }
 
       // If we're about to remove the last observer from the list,
       // then we can remove this observer_list entirely.
       if (list->size() == 1)
         observer_lists_.erase(loop);
     }
     list->RemoveObserver(obs);
 
     // If RemoveObserver is called from a notification, the size will be
     // nonzero.  Instead of deleting here, the NotifyWrapper will delete
@@ skipping 82 matching lines @@
   typedef std::map<MessageLoop*, ObserverList<ObserverType>*> ObserversListMap;
 
   // These are marked mutable to facilitate having NotifyAll be const.
   Lock list_lock_;  // Protects the observer_lists_.
   ObserversListMap observer_lists_;
 
   DISALLOW_EVIL_CONSTRUCTORS(ObserverListThreadSafe);
 };
 
 #endif  // BASE_OBSERVER_LIST_THREADSAFE_H_