Bug 551242 - [OOPP] Hang (caught by hang detector) with flash and alt key. r=bent.

ipc/glue/WindowsMessageLoop.cpp

@@ -217,6 +217,7 @@ ProcessOrDeferMessage(HWND hwnd,
     case WM_NCDESTROY:
     case WM_PARENTNOTIFY:
     case WM_SETFOCUS:
+    case WM_SYSCOMMAND:
     case WM_SHOWWINDOW: // Intentional fall-through.
     case WM_XP_THEMECHANGED: {
       deferred = new DeferredSendMessage(hwnd, uMsg, wParam, lParam);