Adjust use of chrome.storage.managed for Opera

It turns out that chrome.storage.managed is defined for Opera, so the
previous feature detection method did not work.

web/preferences.js

@@ -204,11 +204,15 @@ var Preferences = {
 //      // These preferences can be overridden by the user.
 //      chrome.storage.managed.get(DEFAULT_PREFERENCES, getPreferences);
 //    } else {
-//      // Managed storage not supported, e.g. in Opera.
+//      // Managed storage not supported, e.g. in old Chromium versions.
 //      getPreferences(DEFAULT_PREFERENCES);
 //    }
 //
 //    function getPreferences(defaultPrefs) {
+//      if (chrome.runtime.lastError) {
+//        // Managed storage not supported, e.g. in Opera.
+//        defaultPrefs = DEFAULT_PREFERENCES;
+//      }
 //      chrome.storage.local.get(defaultPrefs, function(readPrefs) {
 //        resolve(readPrefs);
 //      });