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

content/browser/service_worker/service_worker_request_handler.cc

Index: content/browser/service_worker/service_worker_request_handler.cc
diff --git a/content/browser/service_worker/service_worker_request_handler.cc b/content/browser/service_worker/service_worker_request_handler.cc
index f27f24201937655c5d215253b390b93a0a6607ad..505676748008475e7bb0219bdb12dcece570c266 100644
--- a/content/browser/service_worker/service_worker_request_handler.cc
+++ b/content/browser/service_worker/service_worker_request_handler.cc
@@ -22,6 +22,7 @@
 #include "content/public/browser/resource_context.h"
 #include "content/public/common/browser_side_navigation_policy.h"
 #include "content/public/common/child_process_host.h"
+#include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/origin_util.h"
 #include "ipc/ipc_message.h"
@@ -130,8 +131,7 @@ ServiceWorkerRequestHandler::InitializeForNavigationNetworkService(
     scoped_refptr<ResourceRequestBodyImpl> body,
     const base::Callback<WebContents*(void)>& web_contents_getter) {
   DCHECK(IsBrowserSideNavigationEnabled() &&
-         base::CommandLine::ForCurrentProcess()->HasSwitch(
-             switches::kEnableNetworkService));
+         base::FeatureList::IsEnabled(features::kNetworkService));

[Alexei Svitkine (slow), 2017/06/29 13:58:24]

NIt: Seems better to have these as separate DCHECK()'s.

[jam, 2017/06/30 17:21:18]

since I'm just changing flag->feature, I'll leave it to how the original author
preferred to write it.

   DCHECK(navigation_handle_core);
 
   // Create the handler even for insecure HTTP since it's used in the