Re-lock the screen if we're restarted from a previously crashed shell

This way we "fail closed", which is better for security.

See https://bugs.launchpad.net/ubuntu/+source/gdm/+bug/1064584

https://bugzilla.gnome.org/show_bug.cgi?id=691987

js/ui/main.js

@@ -195,6 +195,9 @@ function _initializeUI() {
                               if (keybindingMode == Shell.KeyBindingMode.NONE) {
                                   keybindingMode = Shell.KeyBindingMode.NORMAL;
                               }
+                              if (screenShield) {
+                                  screenShield.lockIfWasLocked();
+                              }
                           });
 }