Only disable request handling for HTTP/S schemes (see issue #3110)

The `--disable-request-handling-for-testing` command-line flag would previously
disable handling of all schemes, including custom schemes and internal schemes
such as devtools.

libcef/browser/net_service/proxy_url_loader_factory.cc

@@ -1313,7 +1313,7 @@ void ProxyURLLoaderFactory::CreateLoaderAndStart(
     return;
   }
 
-  if (DisableRequestHandlingForTesting()) {
+  if (DisableRequestHandlingForTesting() && request.url.SchemeIsHTTPOrHTTPS()) {
     // This is the so-called pass-through, no-op option.
     if (target_factory_) {
       target_factory_->CreateLoaderAndStart(std::move(receiver), request_id,