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

chrome/browser/flag_descriptions.h

Index: chrome/browser/flag_descriptions.h
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 7ded2a4ed5e180d38436cd27379bd8570647c545..c1a282f7b4775d8a2f023c1df5d26549950eb80d 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -242,6 +242,9 @@ extern const char kEnableMidiManagerDynamicInstantiationDescription[];
 extern const char kEnableNavigationTracingName[];
 extern const char kEnableNavigationTracingDescription[];
 
+extern const char kEnableNetworkServiceName[];
+extern const char kEnableNetworkServiceDescription[];
+
 extern const char kEnablePictureInPictureName[];
 extern const char kEnablePictureInPictureDescription[];