Cleanup: Remove NOTIFICATION_EXTENSION_UNLOADED usage in MediaScanManager.

BUG=354046

Review URL: https://codereview.chromium.org/207613002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262295 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/extensions/browser/extension_registry.h b/extensions/browser/extension_registry.h
index 3030c07..26e30159 100644
--- a/extensions/browser/extension_registry.h
+++ b/extensions/browser/extension_registry.h
@@ -36,7 +36,7 @@
     EVERYTHING = (1 << 4) - 1,
   };
 
-  ExtensionRegistry();
+  explicit ExtensionRegistry(content::BrowserContext* browser_context);
   virtual ~ExtensionRegistry();
 
   // Returns the instance for the given |browser_context|.
@@ -137,6 +137,8 @@
 
   ObserverList<ExtensionRegistryObserver> observers_;
 
+  content::BrowserContext* const browser_context_;
+
   DISALLOW_COPY_AND_ASSIGN(ExtensionRegistry);
 };