Move enable extensions define to a build flag.

chrome/browser/ui/browser.cc

Index: chrome/browser/ui/browser.cc
diff --git a/chrome/browser/ui/browser.cc b/chrome/browser/ui/browser.cc
index b5b54d017c63e1e0fe8dcfe336adeb0b12bf1eaa..29fbcd6c4a9750ba5eda19cdf835f600c4faa324 100644
--- a/chrome/browser/ui/browser.cc
+++ b/chrome/browser/ui/browser.cc
@@ -203,6 +203,7 @@
 #include "extensions/common/constants.h"
 #include "extensions/common/extension.h"
 #include "extensions/common/manifest_handlers/background_info.h"
+#include "extensions/features/features.h"
 #include "net/base/filename_util.h"
 #include "net/base/registry_controlled_domains/registry_controlled_domain.h"
 #include "net/cookies/cookie_monster.h"
@@ -2012,7 +2013,7 @@ void Browser::Observe(int type,
   }
 }
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 ///////////////////////////////////////////////////////////////////////////////
 // Browser, extensions::ExtensionRegistryObserver implementation:
 
@@ -2066,7 +2067,7 @@ void Browser::OnExtensionUnloaded(
     }
   }
 }
-#endif  // defined(ENABLE_EXTENSIONS)
+#endif  // BUILDFLAG(ENABLE_EXTENSIONS)
 
 ///////////////////////////////////////////////////////////////////////////////
 // Browser, translate::ContentTranslateDriver::Observer implementation: