Move enable extensions define to a build flag.

chrome/browser/prerender/prerender_manager_factory.cc

Index: chrome/browser/prerender/prerender_manager_factory.cc
diff --git a/chrome/browser/prerender/prerender_manager_factory.cc b/chrome/browser/prerender/prerender_manager_factory.cc
index 895e4f4423aeb16456b837311cf50c9027410334..5466972f19f5ce5aa5f0ab9cedffb9daf856cc0a 100644
--- a/chrome/browser/prerender/prerender_manager_factory.cc
+++ b/chrome/browser/prerender/prerender_manager_factory.cc
@@ -12,8 +12,9 @@
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/sync/profile_sync_service_factory.h"
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
+#include "extensions/features/features.h"
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 #include "extensions/browser/extension_system_provider.h"
 #include "extensions/browser/extensions_browser_client.h"
 #endif
@@ -39,7 +40,7 @@ PrerenderManagerFactory::PrerenderManagerFactory()
     : BrowserContextKeyedServiceFactory(
         "PrerenderManager",
         BrowserContextDependencyManager::GetInstance()) {
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   DependsOn(
       extensions::ExtensionsBrowserClient::Get()->GetExtensionSystemFactory());
 #endif