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

chrome/browser/extensions/extension_service.h

Index: chrome/browser/extensions/extension_service.h
diff --git a/chrome/browser/extensions/extension_service.h b/chrome/browser/extensions/extension_service.h
index 4550f897019a4169ecb01885d755aa6e2fa4de43..77b1e67b905f4117d2968ffc18af3917e1b38636 100644
--- a/chrome/browser/extensions/extension_service.h
+++ b/chrome/browser/extensions/extension_service.h
@@ -31,6 +31,10 @@
 #include "extensions/common/manifest.h"
 #include "sync/api/string_ordinal.h"
 
+#if !defined(ENABLE_EXTENSIONS)
+#error "Extensions must be enabled"
+#endif
+
 class ExtensionSyncService;
 class GURL;
 class HostContentSettingsMap;