Move enable extensions define to a build flag.

chrome/browser/sync/glue/extensions_activity_monitor.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_SYNC_GLUE_EXTENSIONS_ACTIVITY_MONITOR_H_
 #define CHROME_BROWSER_SYNC_GLUE_EXTENSIONS_ACTIVITY_MONITOR_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
+#include "extensions/features/features.h"
 
 namespace syncer {
 class ExtensionsActivity;
 }
 
 namespace browser_sync {
 
 // Observe and record usage of extension bookmark API.
 class ExtensionsActivityMonitor : public content::NotificationObserver {
  public:
   ExtensionsActivityMonitor();
   ~ExtensionsActivityMonitor() override;
 
   // content::NotificationObserver implementation.
   void Observe(int type,
                const content::NotificationSource& source,
                const content::NotificationDetails& details) override;
 
   const scoped_refptr<syncer::ExtensionsActivity>& GetExtensionsActivity();
 
  private:
   scoped_refptr<syncer::ExtensionsActivity> extensions_activity_;
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   // Used only on UI loop.
   content::NotificationRegistrar registrar_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionsActivityMonitor);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_GLUE_EXTENSIONS_ACTIVITY_MONITOR_H_