Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/ui/gtk/gtk_theme_service.cc

Index: chrome/browser/ui/gtk/gtk_theme_service.cc
diff --git a/chrome/browser/ui/gtk/gtk_theme_service.cc b/chrome/browser/ui/gtk/gtk_theme_service.cc
index 814c53584a7dc8bc1f2bd378f635513ccb15e228..07da50ccc311a15661f480da1fa74d8a13213f08 100644
--- a/chrome/browser/ui/gtk/gtk_theme_service.cc
+++ b/chrome/browser/ui/gtk/gtk_theme_service.cc
@@ -367,16 +367,14 @@ bool GtkThemeService::UsingNativeTheme() const {
   return use_gtk_;
 }
 
-void GtkThemeService::Observe(int type,
-                              const content::NotificationSource& source,
-                              const content::NotificationDetails& details) {
-  if ((type == chrome::NOTIFICATION_PREF_CHANGED) &&
-      (*content::Details<std::string>(details).ptr() ==
-          prefs::kUsesSystemTheme)) {
+void GtkThemeService::OnPreferenceChanged(PrefServiceBase* service,
+                                          const std::string& pref_name) {
+  if (pref_name == prefs::kUsesSystemTheme) {
     use_gtk_ = profile()->GetPrefs()->GetBoolean(prefs::kUsesSystemTheme);
-  } else {
-    ThemeService::Observe(type, source, details);
   }
+
+  // ThemeService does not observe prefs changes, so no need to
+  // forward to the base class.
 }
 
 GtkWidget* GtkThemeService::BuildChromeButton() {