Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/ui/cocoa/browser_window_cocoa.mm

Index: chrome/browser/ui/cocoa/browser_window_cocoa.mm
diff --git a/chrome/browser/ui/cocoa/browser_window_cocoa.mm b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
index b15fb7ced4e2a3df4f57e544a28958f5ac1a486d..1c875c6f7327da2e15e2576d9baac8267edfe507 100644
--- a/chrome/browser/ui/cocoa/browser_window_cocoa.mm
+++ b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
@@ -614,21 +614,10 @@ bool BrowserWindowCocoa::GetConstrainedWindowTopY(int* top_y) {
   return false;
 }
 
-void BrowserWindowCocoa::Observe(int type,
-                                 const content::NotificationSource& source,
-                                 const content::NotificationDetails& details) {
-  switch (type) {
-    case chrome::NOTIFICATION_PREF_CHANGED: {
-      const std::string& pref_name =
-          *content::Details<std::string>(details).ptr();
-      DCHECK(pref_name == prefs::kShowBookmarkBar);
-      [controller_ updateBookmarkBarVisibilityWithAnimation:YES];
-      break;
-    }
-    default:
-      NOTREACHED();  // we don't ask for anything else!
-      break;
-  }
+void BrowserWindowCocoa::OnPreferenceChanged(PrefServiceBase* service,
+                                             const std::string& pref_name) {
+  DCHECK(pref_name == prefs::kShowBookmarkBar);
+  [controller_ updateBookmarkBarVisibilityWithAnimation:YES];
 }
 
 extensions::ActiveTabPermissionGranter*