MD Settings: Fix some tests that break when run in Vulcanized mode.

chrome/browser/resources/settings/prefs/prefs.js

Index: chrome/browser/resources/settings/prefs/prefs.js
diff --git a/chrome/browser/resources/settings/prefs/prefs.js b/chrome/browser/resources/settings/prefs/prefs.js
index 4988cf0ea05699673893151be6e5b917d65e5b7a..15df5f2471f90b3c6d5c213c5b5b47f545930183 100644
--- a/chrome/browser/resources/settings/prefs/prefs.js
+++ b/chrome/browser/resources/settings/prefs/prefs.js
@@ -147,11 +147,17 @@
     /** @type {SettingsPrivate} */
     settingsApi_: /** @type {SettingsPrivate} */(chrome.settingsPrivate),
 
+    /** @override */
     created: function() {
       if (!CrSettingsPrefs.deferInitialization)
         this.initialize();
     },
 
+    /** @override */
+    detached: function() {
+      CrSettingsPrefs.resetForTesting();
+    },
+
     /**
      * @param {SettingsPrivate=} opt_settingsApi SettingsPrivate implementation
      *     to use (chrome.settingsPrivate by default).