Arc ToS Screen: Avoid spammy error log

Avoid causing an error log message from the ARC ToS screen:

  [ERROR:CONSOLE(219)] "Unexpected condition on chrome://oobe/gaia-signin:
  Could not find value for languageList",
  source: chrome://resources/js/load_time_data.js (219)

Bug: 964121
Test: manual - go to Login screen and verify there's no error in the console
Change-Id: Ia9c66844817ae45e199c3a35c9ff45a720b79b43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615942Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660815}

chrome/browser/resources/chromeos/login/screen_arc_terms_of_service.js

@@ -33,11 +33,14 @@ login.createScreen('ArcTermsOfServiceScreen', 'arc-tos', function() {
      * @private
      */
     getCurrentLanguage_: function() {
-      var languageList = loadTimeData.getValue('languageList');
-      if (languageList) {
-        var language = getSelectedValue(languageList);
-        if (language) {
-          return language;
+      const LANGUAGE_LIST_ID = 'languageList';
+      if (loadTimeData.valueExists(LANGUAGE_LIST_ID)) {
+        var languageList = loadTimeData.getValue(LANGUAGE_LIST_ID);
+        if (languageList) {
+          var language = getSelectedValue(languageList);
+          if (language) {
+            return language;
+          }
         }
       }
       return navigator.language;