Make the SystemMonitor observer list thread safe....

base/observer_list_threadsafe.h

Index: base/observer_list_threadsafe.h
===================================================================
--- base/observer_list_threadsafe.h	(revision 4449)
+++ base/observer_list_threadsafe.h	(working copy)
@@ -81,11 +81,15 @@
   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.