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

content/public/common/content_features.cc

Index: content/public/common/content_features.cc
diff --git a/content/public/common/content_features.cc b/content/public/common/content_features.cc
index 7f741009aed056a76e6a8628829472e4b84391dc..4b845325a67ba0a77b66c099d3f36d0663f111ac 100644
--- a/content/public/common/content_features.cc
+++ b/content/public/common/content_features.cc
@@ -163,6 +163,10 @@ const base::Feature kMemoryCoordinator {
   "MemoryCoordinator", base::FEATURE_DISABLED_BY_DEFAULT
 };
 
+// Enables the network service.
+const base::Feature kNetworkService{"NetworkService",
+                                    base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Kill switch for Web Notification content images.
 const base::Feature kNotificationContentImage{"NotificationContentImage",
                                               base::FEATURE_ENABLED_BY_DEFAULT};