[Extensions] Move [Chrome]ExtensionTestNotificationObserver into extensions::

extensions/test/extension_test_notification_observer.h

Index: extensions/test/extension_test_notification_observer.h
diff --git a/extensions/test/extension_test_notification_observer.h b/extensions/test/extension_test_notification_observer.h
index b09a340ac307eb08588b0a1cde0f9e6cddce55d3..655084eb012e27f6f061643b054fffa4339794e3 100644
--- a/extensions/test/extension_test_notification_observer.h
+++ b/extensions/test/extension_test_notification_observer.h
@@ -24,7 +24,6 @@ class WindowedNotificationObserver;
 
 namespace extensions {
 class ProcessManager;
-}
 
 // Test helper class for observing extension-related events.
 class ExtensionTestNotificationObserver : public content::NotificationObserver {
@@ -139,4 +138,6 @@ class ExtensionTestNotificationObserver : public content::NotificationObserver {
   DISALLOW_COPY_AND_ASSIGN(ExtensionTestNotificationObserver);
 };
 
+}  // namespace extensions
+
 #endif  // EXTENSIONS_TEST_EXTENSION_TEST_NOTIFICATION_OBSERVER_H_