Move enable extensions define to a build flag.

chrome/browser/ui/webui/policy_ui_handler.h

Index: chrome/browser/ui/webui/policy_ui_handler.h
diff --git a/chrome/browser/ui/webui/policy_ui_handler.h b/chrome/browser/ui/webui/policy_ui_handler.h
index 7a59b813db57e62e17ee59f13cae19650565ceca..0b34eac7c5f9e448c2c2ea2e12dace34d0c8e7df 100644
--- a/chrome/browser/ui/webui/policy_ui_handler.h
+++ b/chrome/browser/ui/webui/policy_ui_handler.h
@@ -20,8 +20,9 @@
 #include "content/public/browser/web_ui.h"
 #include "content/public/browser/web_ui_data_source.h"
 #include "content/public/browser/web_ui_message_handler.h"
+#include "extensions/features/features.h"
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 #include "extensions/browser/extension_registry_observer.h"
 #endif
 
@@ -34,7 +35,7 @@ class CloudPolicyStatusProvider;
 
 // The JavaScript message handler for the chrome://policy page.
 class PolicyUIHandler : public content::WebUIMessageHandler,
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
                         public extensions::ExtensionRegistryObserver,
 #endif
                         public policy::PolicyService::Observer,
@@ -54,7 +55,7 @@ class PolicyUIHandler : public content::WebUIMessageHandler,
   // content::WebUIMessageHandler implementation.
   void RegisterMessages() override;
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   // extensions::ExtensionRegistryObserver implementation.
   void OnExtensionLoaded(content::BrowserContext* browser_context,
                          const extensions::Extension* extension) override;