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

chrome/browser/flag_descriptions.cc

Index: chrome/browser/flag_descriptions.cc
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index b3e61a041f24a436a45bfe53548c298880210df5..7d3d8769b826684a4288cacb1b690c2c33141538 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -1067,6 +1067,12 @@ const char kEnableNavigationTracingDescription[] =
     "flag. The trace may include personally identifiable information (PII) "
     "such as the titles and URLs of websites you visit.";
 
+const char kEnableNetworkServiceName[] = "Enable network service";
+
+const char kEnableNetworkServiceDescription[] =
+    "Enables the network service, which makes network requests through a "
+    "separate service. Note: most features don't work with this yet.";
+
 const char kEnablePictureInPictureName[] = "Enable picture in picture.";
 
 const char kEnablePictureInPictureDescription[] =