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

tools/metrics/histograms/enums.xml

Index: tools/metrics/histograms/enums.xml
diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index b27765cf32c449d856f235ec6f670305ad61e286..941eb4e1181c21a8f5b2f887ac2bdb1d40758019 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -22825,6 +22825,7 @@ from previous Chrome versions.
   <int value="598827460" label="enable-roboto-font-ui"/>
   <int value="602117675" label="NTPBookmarkSuggestions:enabled"/>
   <int value="603326800" label="UsePasswordSeparatedSigninFlow:enabled"/>
+  <int value="603988014" label="NetworkService:enabled"/>
   <int value="605150752" label="WebUSB:disabled"/>
   <int value="606288133" label="enable-print-preview-register-promos"/>
   <int value="606512202" label="AutofillCreditCardPopupLayout:enabled"/>
@@ -23068,6 +23069,7 @@ from previous Chrome versions.
   <int value="1386300777"
       label="OmniboxUIExperimentHideSuggestionUrlTrivialSubdomains:disabled"/>
   <int value="1389729816" label="data-reduction-proxy-lo-fi"/>
+  <int value="1397069250" label="NetworkService:disabled"/>
   <int value="1405459667" label="enable-fast-text-autosizing"/>
   <int value="1406354320" label="MacViewsWebUIDialogs:enabled"/>
   <int value="1407625309"