Remove InstallTracker's extension load, unload observer

extensions/browser/extension_registry_observer.h

Index: extensions/browser/extension_registry_observer.h
diff --git a/extensions/browser/extension_registry_observer.h b/extensions/browser/extension_registry_observer.h
index 7cb524ececac01ced97439a95eda529489447df2..01bc19a4f3e2c09c73296958c4a5b6c715b2392c 100644
--- a/extensions/browser/extension_registry_observer.h
+++ b/extensions/browser/extension_registry_observer.h
@@ -62,6 +62,9 @@ class ExtensionRegistryObserver {
   // any of the ExtensionRegistry sets (enabled, disabled, etc.).
   virtual void OnExtensionUninstalled(content::BrowserContext* browser_context,
                                       const Extension* extension) {}
+
+  // Notifies observers that the observed object is going away.
+  virtual void OnShutdown() {}
 };
 
 }  // namespace extensions