Bug 1369002 - Stop warning if the mainThread is null in GetCurrentThread. r=froydnj

xpcom/threads/nsThreadManager.cpp

@@ -328,7 +328,7 @@ NS_IMETHODIMP
 nsThreadManager::GetCurrentThread(nsIThread** aResult)
 {
   // Keep this functioning during Shutdown
-  if (NS_WARN_IF(!mMainThread)) {
+  if (!mMainThread) {
     return NS_ERROR_NOT_INITIALIZED;
   }
   *aResult = GetCurrentThread();