Enable the mojo-based serial API in the renderer behind a flag.

extensions/browser/service_registration_manager.cc

Index: extensions/browser/service_registration_manager.cc
diff --git a/extensions/browser/service_registration_manager.cc b/extensions/browser/service_registration_manager.cc
index 1a727bf73a3b943b241fbe423261d267ce0905b2..49acf45b4bd9598587d29e820d4d819922aa9466 100644
--- a/extensions/browser/service_registration_manager.cc
+++ b/extensions/browser/service_registration_manager.cc
@@ -45,6 +45,10 @@ ServiceRegistrationManager* ServiceRegistrationManager::GetSharedInstance() {
   return g_lazy_instance.Pointer();
 }
 
+void ServiceRegistrationManager::ClearOverridesForTest() {
+  test_factories_.clear();
+}
+
 void ServiceRegistrationManager::AddToRenderFrame(
     content::RenderFrameHost* render_frame_host) {
   content::BrowserContext* context =
@@ -75,7 +79,12 @@ void ServiceRegistrationManager::AddToRenderFrame(
     if (ExtensionAPI::GetSharedInstance()
             ->IsAvailable(factory.first, extension, context_type, site_url)
             .is_available()) {
-      factory.second->Register(render_frame_host->GetServiceRegistry());
+      auto test_factory = test_factories_.find(factory.second->GetName());
+      if (test_factory != test_factories_.end()) {
+        test_factory->second->Register(render_frame_host->GetServiceRegistry());
+      } else {
+        factory.second->Register(render_frame_host->GetServiceRegistry());
+      }
     }
   }
 }