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

Review URL: https://codereview.chromium.org/645043003

Cr-Commit-Position: refs/heads/master@{#300053}
diff --git a/extensions/browser/extension_registry.h b/extensions/browser/extension_registry.h
index 27ef4af1..9221a77b 100644
--- a/extensions/browser/extension_registry.h
+++ b/extensions/browser/extension_registry.h
@@ -14,6 +14,10 @@
 #include "extensions/browser/uninstall_reason.h"
 #include "extensions/common/extension_set.h"
 
+#if !defined(ENABLE_EXTENSIONS)
+#error "Extensions must be enabled"
+#endif
+
 namespace content {
 class BrowserContext;
 }