Move enable extensions define to a build flag.

chrome/browser/sync/glue/extensions_activity_monitor.cc

Index: chrome/browser/sync/glue/extensions_activity_monitor.cc
diff --git a/chrome/browser/sync/glue/extensions_activity_monitor.cc b/chrome/browser/sync/glue/extensions_activity_monitor.cc
index a3c735e046ca738fbaba53564ba2046477ef6f56..60d2284a3915396162138052461cf0079aece541 100644
--- a/chrome/browser/sync/glue/extensions_activity_monitor.cc
+++ b/chrome/browser/sync/glue/extensions_activity_monitor.cc
@@ -6,8 +6,9 @@
 
 #include "components/sync/base/extensions_activity.h"
 #include "content/public/browser/browser_thread.h"
+#include "extensions/features/features.h"
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/extensions/api/bookmarks/bookmarks_api.h"
 #include "content/public/browser/notification_service.h"
@@ -26,7 +27,7 @@ ExtensionsActivityMonitor::ExtensionsActivityMonitor()
   // the fly so there is no reliable object to point to (same problem if we
   // wanted to use the string name).  Thus, we use all sources and filter in
   // Observe.
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   registrar_.Add(this,
                  extensions::NOTIFICATION_EXTENSION_BOOKMARKS_API_INVOKED,
                  content::NotificationService::AllSources());
@@ -41,7 +42,7 @@ void ExtensionsActivityMonitor::Observe(
     int type,
     const content::NotificationSource& source,
     const content::NotificationDetails& details) {
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   DCHECK_EQ(extensions::NOTIFICATION_EXTENSION_BOOKMARKS_API_INVOKED, type);
   const extensions::Extension* extension =