Cleanup: Prevent usage of various extension headers when extensions support is not compiled in.

extensions/browser/notification_types.h

Index: extensions/browser/notification_types.h
diff --git a/extensions/browser/notification_types.h b/extensions/browser/notification_types.h
index d0b2c56caad833426630ed12bb6604761fa53f04..6eee02e235b6e2f82c7c6d55c34b8e8498cb18f0 100644
--- a/extensions/browser/notification_types.h
+++ b/extensions/browser/notification_types.h
@@ -7,12 +7,17 @@
 
 #include "content/public/browser/notification_types.h"
 
+#if !defined(ENABLE_EXTENSIONS)
+#error "Extensions must be enabled"
+#endif
+
 namespace extensions {
 
 // Only notifications fired by the extensions module should be here. The
 // extensions module should not listen to notifications fired by the
 // embedder.
 enum NotificationType {
+  // WARNING: This need to match chrome/browser/chrome_notification_types.h.
   NOTIFICATION_EXTENSIONS_START = content::NOTIFICATION_CONTENT_END,
 
   // Sent when a CrxInstaller finishes. Source is the CrxInstaller that