fix: flash plugin (#22110)

* fix: flash plugin

Fixes #20744

* cleanup

* fix linting issue

Co-authored-by: t57ser <seve@live.at>

shell/browser/net/system_network_context_manager.cc

@@ -11,6 +11,7 @@
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/net/chrome_mojo_proxy_resolver_factory.h"
 #include "content/public/browser/browser_thread.h"
+#include "content/public/browser/cors_exempt_headers.h"
 #include "content/public/browser/network_service_instance.h"
 #include "content/public/common/content_features.h"
 #include "content/public/common/service_names.mojom.h"
@@ -154,6 +155,10 @@ SystemNetworkContextManager::CreateDefaultNetworkContextParams() {
   network::mojom::NetworkContextParamsPtr network_context_params =
       network::mojom::NetworkContextParams::New();
 
+  // This is required to avoid blocking X-Requested-With headers sent by PPAPI
+  // plugins, more info crbug.com/940331
+  content::UpdateCorsExemptHeader(network_context_params.get());
+
   network_context_params->enable_brotli = true;
 
   network_context_params->enable_referrers = true;