Flatten threads: fix feature in Firefox

The feature didn't work in Firefox because we referenced chrome, which
doesn't exist in the main world in Firefox. This CL fixes this issue.

Change-Id: I5cc3363205fb0e60d3b595394025ff123feb6c96
diff --git a/src/common/litI18nUtils.js b/src/common/litI18nUtils.js
index 1f95baa..df502ab 100644
--- a/src/common/litI18nUtils.js
+++ b/src/common/litI18nUtils.js
@@ -16,7 +16,7 @@
 
   #setUpLitL10n() {
     let pLocale;
-    if (chrome.i18n) {
+    if (typeof chrome !== 'undefined' && chrome.i18n) {
       pLocale = Promise.resolve(chrome.i18n.getUILanguage());
     } else {
       pLocale = this.#MWI18nClient.getUILanguage();