Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/content_settings/content_settings_pref_provider.cc

Index: chrome/browser/content_settings/content_settings_pref_provider.cc
diff --git a/chrome/browser/content_settings/content_settings_pref_provider.cc b/chrome/browser/content_settings/content_settings_pref_provider.cc
index 8b4887e7e0bf371a1780e9350539499918e35ac0..e5ad337a5d9d2231b5dce62913b37d961433a93c 100644
--- a/chrome/browser/content_settings/content_settings_pref_provider.cc
+++ b/chrome/browser/content_settings/content_settings_pref_provider.cc
@@ -217,15 +217,11 @@ void PrefProvider::ClearAllContentSettingsRules(
                   std::string());
 }
 
-void PrefProvider::Observe(
-    int type,
-    const content::NotificationSource& source,
-    const content::NotificationDetails& details) {
+void PrefProvider::OnPreferenceChanged(PrefServiceBase* service,
+                                       const std::string& name) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-  DCHECK_EQ(chrome::NOTIFICATION_PREF_CHANGED, type);
-  DCHECK_EQ(content::Source<PrefService>(source).ptr(), prefs_);
-  DCHECK_EQ(std::string(prefs::kContentSettingsPatternPairs),
-            *content::Details<std::string>(details).ptr());
+  DCHECK_EQ(prefs_, service);
+  DCHECK_EQ(std::string(prefs::kContentSettingsPatternPairs), name);
 
   if (updating_preferences_)
     return;