BatteryManager.getBattery(): gracefully bail on context-detached use.

LayoutTests/vibration/vibration-detached-no-crash.html

Index: LayoutTests/vibration/vibration-detached-no-crash.html
diff --git a/LayoutTests/vibration/vibration-detached-no-crash.html b/LayoutTests/vibration/vibration-detached-no-crash.html
index 46b12de5a54a8e51b7acdd480115bf094bb626f8..7e0de30d42831b7ab5aec3aa758a641a9c50fc8c 100644
--- a/LayoutTests/vibration/vibration-detached-no-crash.html
+++ b/LayoutTests/vibration/vibration-detached-no-crash.html
@@ -23,7 +23,7 @@ if (window.testRunner) {
     testRunner.dumpAsText();
     testRunner.setCanOpenWindows();
 }
-w = window.open('resources/vibration-detached-no-crash-new-window.html');
+w = window.open('../resources/window-postmessage-open-close.html');
 window.addEventListener("message", processMessage, false);
 </script>
 </body>