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

This also changes how it's now enabled: --enable-features=network-service

BUG=598073
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2960703003
Cr-Commit-Position: refs/heads/master@{#484295}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 7ded2a4..c1a282f7 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -242,6 +242,9 @@
 extern const char kEnableNavigationTracingName[];
 extern const char kEnableNavigationTracingDescription[];
 
+extern const char kEnableNetworkServiceName[];
+extern const char kEnableNetworkServiceDescription[];
+
 extern const char kEnablePictureInPictureName[];
 extern const char kEnablePictureInPictureDescription[];