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/download_manager_delegate.cc

@@ -12,7 +12,9 @@ namespace cef {
 // static
 std::unique_ptr<cef::DownloadManagerDelegate> DownloadManagerDelegate::Create(
     content::DownloadManager* download_manager) {
+#if BUILDFLAG(ENABLE_ALLOY_BOOTSTRAP)
   REQUIRE_CHROME_RUNTIME();
+#endif
   return std::make_unique<CefDownloadManagerDelegateImpl>(
       download_manager, /*alloy_bootstrap=*/false);
 }