Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/ui/webui/ntp/ntp_resource_cache.cc

Index: chrome/browser/ui/webui/ntp/ntp_resource_cache.cc
diff --git a/chrome/browser/ui/webui/ntp/ntp_resource_cache.cc b/chrome/browser/ui/webui/ntp/ntp_resource_cache.cc
index 4d88b4db3cc7f85d0021457efe3c891a00eb9e73..8277c54d66441e2050e282b2bbfef6db5b4178d8 100644
--- a/chrome/browser/ui/webui/ntp/ntp_resource_cache.cc
+++ b/chrome/browser/ui/webui/ntp/ntp_resource_cache.cc
@@ -219,12 +219,6 @@ void NTPResourceCache::Observe(int type,
     new_tab_html_ = NULL;
     new_tab_incognito_css_ = NULL;
     new_tab_css_ = NULL;
-  } else if (chrome::NOTIFICATION_PREF_CHANGED == type) {
-    // A change occurred to one of the preferences we care about, so flush the
-    // cache.
-    new_tab_incognito_html_ = NULL;
-    new_tab_html_ = NULL;
-    new_tab_css_ = NULL;
   } else if (chrome::NOTIFICATION_NTP_BACKGROUND_THEME_Y_POS_CHANGED == type) {
     css_background_y_pos_ = *(content::Details<int>(details).ptr());
     if (new_tab_css_.get())
@@ -234,6 +228,15 @@ void NTPResourceCache::Observe(int type,
   }
 }
 
+void NTPResourceCache::OnPreferenceChanged(PrefServiceBase* service,
+                                           const std::string& pref_name) {
+  // A change occurred to one of the preferences we care about, so flush the
+  // cache.
+  new_tab_incognito_html_ = NULL;
+  new_tab_html_ = NULL;
+  new_tab_css_ = NULL;
+}
+
 void NTPResourceCache::CreateNewTabIncognitoHTML() {
   DictionaryValue localized_strings;
   localized_strings.SetString("title",