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/browser_context.cc

@@ -366,6 +366,7 @@ void CefBrowserContext::ClearSchemeHandlerFactories() {
                                iothread_state_));
 }
 
+#if BUILDFLAG(ENABLE_ALLOY_BOOTSTRAP)
 void CefBrowserContext::LoadExtension(
     const CefString& root_directory,
     CefRefPtr<CefDictionaryValue> manifest,
@@ -392,6 +393,7 @@ bool CefBrowserContext::UnloadExtension(const CefString& extension_id) {
   NOTIMPLEMENTED();
   return false;
 }
+#endif  // BUILDFLAG(ENABLE_ALLOY_BOOTSTRAP)
 
 network::mojom::NetworkContext* CefBrowserContext::GetNetworkContext() {
   CEF_REQUIRE_UIT();