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

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 766dd5229fd6afc1dccab5262fa899fa3a085eac..9a3d323a86d213b718f11c9fe0b9aa0762f3b362 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -4344,8 +4344,7 @@ void RenderFrameImpl::WillSendRequest(blink::WebURLRequest& request) {
   }
 
   // TODO: generalize how non-network schemes are sent to the renderer and used.
-  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
-          switches::kEnableNetworkService)) {
+  if (base::FeatureList::IsEnabled(features::kNetworkService)) {
     if (request.Url().ProtocolIs(url::kBlobScheme)) {
       extra_data->set_url_loader_factory_override(
           RenderThreadImpl::current()->GetBlobURLLoaderFactory());