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 9a8c6884635fc8ee7cfe8af50565c3d0fe5bac2a..ef099c51ccefca9f039a6386d2a144595ea9e6e5 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -22964,6 +22964,7 @@ 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"/>
@@ -23207,6 +23208,7 @@ 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"