Add a feature flag for network service and about:flags entry for it.

content/browser/service_manager/service_manager_context.cc

Index: content/browser/service_manager/service_manager_context.cc
diff --git a/content/browser/service_manager/service_manager_context.cc b/content/browser/service_manager/service_manager_context.cc
index 18d7950b0bcc81fdf650ad98efa7292827b47ff3..b0056bba3a2cf2e652488dda4dfb380528226a4c 100644
--- a/content/browser/service_manager/service_manager_context.cc
+++ b/content/browser/service_manager/service_manager_context.cc
@@ -30,6 +30,7 @@
 #include "content/public/browser/utility_process_host.h"
 #include "content/public/browser/utility_process_host_client.h"
 #include "content/public/common/content_client.h"
+#include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/service_manager_connection.h"
 #include "content/public/common/service_names.mojom.h"
@@ -354,8 +355,7 @@ ServiceManagerContext::ServiceManagerContext() {
       ->RegisterUnsandboxedOutOfProcessServices(&unsandboxed_services);
 
   bool network_service_enabled =
-      base::CommandLine::ForCurrentProcess()->HasSwitch(
-          switches::kEnableNetworkService);
+      base::FeatureList::IsEnabled(features::kNetworkService);
   if (network_service_enabled) {
     unsandboxed_services.insert(
         std::make_pair(content::mojom::kNetworkServiceName,