Fix minor bug in console_inject.js

If the autorefreshlist experiment is not enabled, the update prompt
shouldn't be hidden when exiting thread lists since it doesn't exist.
This triggered an error when this experiment wasn't enabled (although it
didn't have any effect except showing an error).

Change-Id: I4b3622188c8c2c17dfd25bac2bccd406ee2d5e95
diff --git a/src/content_scripts/console_inject.js b/src/content_scripts/console_inject.js
index dc3e270..9070902 100644
--- a/src/content_scripts/console_inject.js
+++ b/src/content_scripts/console_inject.js
@@ -823,7 +823,8 @@
 
 function handleRemovedNode(node) {
   // Remove snackbar when exiting thread list view
-  if ('tagName' in node && node.tagName == 'EC-THREAD-LIST') {
+  if (options.autorefreshlist && 'tagName' in node &&
+      node.tagName == 'EC-THREAD-LIST') {
     autoRefresh.hideUpdatePrompt();
   }
 }