Bug 1504356 - Lock ThreadListMutex before accessing thread list. r=kmag

--HG--
extra : rebase_source : 728653b7568e8a602616522f38f1b6d68290fb55

xpcom/threads/nsThread.cpp

@@ -444,12 +444,10 @@ nsThread::AddToThreadList()
 void
 nsThread::MaybeRemoveFromThreadList()
 {
+  OffTheBooksMutexAutoLock mal(ThreadListMutex());
   if (isInList()) {
-    OffTheBooksMutexAutoLock mal(ThreadListMutex());
-    if (isInList()) {
-      sActiveThreads--;
-      removeFrom(ThreadList());
-    }
+    sActiveThreads--;
+    removeFrom(ThreadList());
   }
 }