Move enable extensions define to a build flag.

chrome/browser/sync/sessions/notification_service_sessions_router.cc

Index: chrome/browser/sync/sessions/notification_service_sessions_router.cc
diff --git a/chrome/browser/sync/sessions/notification_service_sessions_router.cc b/chrome/browser/sync/sessions/notification_service_sessions_router.cc
index 324591975c3a73bd27cbe3a4df00d396a6866535..836ce30450baf8cbeb775c0e8f0640fe07d2881a 100644
--- a/chrome/browser/sync/sessions/notification_service_sessions_router.cc
+++ b/chrome/browser/sync/sessions/notification_service_sessions_router.cc
@@ -22,6 +22,7 @@
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_source.h"
 #include "content/public/browser/web_contents.h"
+#include "extensions/features/features.h"
 
 #if BUILDFLAG(ANDROID_JAVA_UI)
 #include "chrome/browser/android/tab_android.h"
@@ -32,7 +33,7 @@
 #include "chrome/browser/supervised_user/supervised_user_service_factory.h"
 #endif
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 #include "chrome/browser/extensions/tab_helper.h"
 #endif
 
@@ -76,7 +77,7 @@ NotificationServiceSessionsRouter::NotificationServiceSessionsRouter(
       content::NotificationService::AllSources());
   registrar_.Add(this, content::NOTIFICATION_NAV_ENTRY_COMMITTED,
       content::NotificationService::AllSources());
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   registrar_.Add(this,
       chrome::NOTIFICATION_TAB_CONTENTS_APPLICATION_EXTENSION_CHANGED,
       content::NotificationService::AllSources());
@@ -147,7 +148,7 @@ void NotificationServiceSessionsRouter::Observe(
         return;
       break;
     }
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
     case chrome::NOTIFICATION_TAB_CONTENTS_APPLICATION_EXTENSION_CHANGED: {
       extensions::TabHelper* extension_tab_helper =
           content::Source<extensions::TabHelper>(source).ptr();