Revert 170483 - Remove PrefObserver usages, batch 12.

chrome/browser/ui/cocoa/browser_window_cocoa.mm

Index: chrome/browser/ui/cocoa/browser_window_cocoa.mm
===================================================================
--- chrome/browser/ui/cocoa/browser_window_cocoa.mm	(revision 170518)
+++ chrome/browser/ui/cocoa/browser_window_cocoa.mm	(working copy)
@@ -89,10 +89,7 @@
     attention_request_id_(0) {
 
   pref_change_registrar_.Init(browser_->profile()->GetPrefs());
-  pref_change_registrar_.Add(
-      prefs::kShowBookmarkBar,
-      base::Bind(&BrowserWindowCocoa::OnShowBookmarkBarChanged,
-                 base::Unretained(this)));
+  pref_change_registrar_.Add(prefs::kShowBookmarkBar, this);
   gfx::Rect bounds;
   chrome::GetSavedWindowBoundsAndShowState(browser_,
                                            &bounds,
@@ -617,7 +614,9 @@
   return false;
 }
 
-void BrowserWindowCocoa::OnShowBookmarkBarChanged() {
+void BrowserWindowCocoa::OnPreferenceChanged(PrefServiceBase* service,
+                                             const std::string& pref_name) {
+  DCHECK(pref_name == prefs::kShowBookmarkBar);
   [controller_ updateBookmarkBarVisibilityWithAnimation:YES];
 }