Hide ARC VPN behind a feature flag

The UI work will be completed in the M64 timeframe, so for M63 we
will require ARC VPN to be manually enabled via chrome://flags#arc-vpn

BUG=696865
TEST=manually test in both Enabled and Disabled state

Change-Id: I5e362f2437629980c69dcfcef60a11786131e986
Reviewed-on: https://chromium-review.googlesource.com/736817
Commit-Queue: Kevin Cernekee <[email protected]>
Reviewed-by: Elijah Taylor <[email protected]>
Reviewed-by: Steven Bennetts <[email protected]>
Cr-Commit-Position: refs/heads/master@{#511507}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 3308fcb..d45baae 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1446,6 +1446,9 @@
 extern const char kArcNativeBridgeExperimentName[];
 extern const char kArcNativeBridgeExperimentDescription[];
 
+extern const char kArcVpnName[];
+extern const char kArcVpnDescription[];
+
 extern const char kAshEnableUnifiedDesktopName[];
 extern const char kAshEnableUnifiedDesktopDescription[];