Add GN arg to disable Alloy bootstrap (see #3681, see #3685)

Set enable_alloy_bootstrap=false to build with Alloy bootstrap code
removed. Extension API is documented as deprecated in comments but
not compiled out with this arg.

libcef/browser/alloy/alloy_download_util.h

@@ -6,6 +6,8 @@
 #define CEF_LIBCEF_BROWSER_ALLOY_ALLOY_DOWNLOAD_UTIL_H_
 #pragma once
 
+#include "cef/libcef/features/features.h"
+
 class DownloadPrefs;
 
 namespace content {