Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/ui/gtk/browser_toolbar_gtk.cc

Index: chrome/browser/ui/gtk/browser_toolbar_gtk.cc
diff --git a/chrome/browser/ui/gtk/browser_toolbar_gtk.cc b/chrome/browser/ui/gtk/browser_toolbar_gtk.cc
index d272e1e85cce0657fd6dc187a07a3ff3987b77d9..b747c75eb87367f808ec6054f0dbe2af184eb754 100644
--- a/chrome/browser/ui/gtk/browser_toolbar_gtk.cc
+++ b/chrome/browser/ui/gtk/browser_toolbar_gtk.cc
@@ -368,9 +368,7 @@ bool BrowserToolbarGtk::GetAcceleratorForCommandId(
 void BrowserToolbarGtk::Observe(int type,
                                 const content::NotificationSource& source,
                                 const content::NotificationDetails& details) {
-  if (type == chrome::NOTIFICATION_PREF_CHANGED) {
-    NotifyPrefChanged(content::Details<std::string>(details).ptr());
-  } else if (type == chrome::NOTIFICATION_BROWSER_THEME_CHANGED) {
+  if (type == chrome::NOTIFICATION_BROWSER_THEME_CHANGED) {
     // Update the spacing around the menu buttons
     bool use_gtk = theme_service_->UsingNativeTheme();
     int border = use_gtk ? 0 : 2;
@@ -426,6 +424,11 @@ void BrowserToolbarGtk::Observe(int type,
   }
 }
 
+void BrowserToolbarGtk::OnPreferenceChanged(PrefServiceBase* service,
+                                            const std::string& pref_name) {
+  NotifyPrefChanged(pref_name);
+}
+
 // BrowserToolbarGtk, public ---------------------------------------------------
 
 void BrowserToolbarGtk::UpdateWebContents(WebContents* contents,
@@ -650,15 +653,14 @@ void BrowserToolbarGtk::OnDragDataReceived(GtkWidget* widget,
     home_page_.SetValue(url.spec());
 }
 
-void BrowserToolbarGtk::NotifyPrefChanged(const std::string* pref) {
-  if (!pref || *pref == prefs::kShowHomeButton) {
+void BrowserToolbarGtk::NotifyPrefChanged(const std::string& pref) {
+  if (pref == prefs::kShowHomeButton) {
     bool visible = show_home_button_.GetValue() && !ShouldOnlyShowLocation();
     gtk_widget_set_visible(home_->widget(), visible);
   }
 
-  if (!pref ||
-      *pref == prefs::kHomePage ||
-      *pref == prefs::kHomePageIsNewTabPage)
+  if (pref == prefs::kHomePage ||
+      pref == prefs::kHomePageIsNewTabPage)
     SetUpDragForHomeButton(!home_page_.IsManaged() &&
                            !home_page_is_new_tab_page_.IsManaged());
 }