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 183f225d462502060f4facf17cb41ce7f3203f01..e9022eb3e96e306c1440f8dd69c33a18dd0a5190 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -1060,6 +1060,12 @@ 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[] =