Move enable extensions define to a build flag.

chrome/browser/ui/browser.h

Index: chrome/browser/ui/browser.h
diff --git a/chrome/browser/ui/browser.h b/chrome/browser/ui/browser.h
index 8be65005a537e8cc74781e2b2776f3aa06039847..6cdff54b676fd12f703c9128fc8f9c640a2aca19 100644
--- a/chrome/browser/ui/browser.h
+++ b/chrome/browser/ui/browser.h
@@ -45,13 +45,14 @@
 #include "content/public/browser/page_navigator.h"
 #include "content/public/browser/web_contents_delegate.h"
 #include "content/public/common/page_zoom.h"
+#include "extensions/features/features.h"
 #include "ui/base/page_transition_types.h"
 #include "ui/base/ui_base_types.h"
 #include "ui/base/window_open_disposition.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/shell_dialogs/select_file_dialog.h"
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 #include "extensions/browser/extension_registry_observer.h"
 #endif
 
@@ -115,7 +116,7 @@ class Browser : public TabStripModelObserver,
                 public zoom::ZoomObserver,
                 public content::PageNavigator,
                 public content::NotificationObserver,
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
                 public extensions::ExtensionRegistryObserver,
 #endif
                 public translate::ContentTranslateDriver::Observer,
@@ -729,7 +730,7 @@ class Browser : public TabStripModelObserver,
                const content::NotificationSource& source,
                const content::NotificationDetails& details) override;
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   // Overridden from extensions::ExtensionRegistryObserver:
   void OnExtensionUninstalled(content::BrowserContext* browser_context,
                               const extensions::Extension* extension,
@@ -871,7 +872,7 @@ class Browser : public TabStripModelObserver,
 
   content::NotificationRegistrar registrar_;
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   ScopedObserver<extensions::ExtensionRegistry,
                  extensions::ExtensionRegistryObserver>
       extension_registry_observer_;