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

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 855a0ffa4c5b42d3a1e78e4a308caa715a347014..5926e15f897ecffd76749cd28fa0f4132024020a 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -3104,6 +3104,10 @@ const FeatureEntry kFeatureEntries[] = {
      FEATURE_VALUE_TYPE(features::kBrowserTouchBar)},
 #endif  // defined(OS_MACOSX)
 
+    {"network-service", flag_descriptions::kEnableNetworkServiceName,
+     flag_descriptions::kEnableNetworkServiceDescription, kOsAll,
+     FEATURE_VALUE_TYPE(features::kNetworkService)},
+
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms/enums.xml. See note in
     // enums.xml and don't forget to run AboutFlagsHistogramTest unit test.