Add flag so shill sandboxing can be force enabled/disabled.

This flag allows users to work around a shill-related breakage due to
sandboxing, by forcing the feature disabled. It also allows testers to
toggle the feature during testing without needing to modify
/etc/chrome-dev.conf. Having this flag in the chrome://flags will allow
a user to manually enable/disable shill sandboxing without needing a
VariationsService update, which might be infeasible if network
connectivity is lost.

BUG=chromium:649417

Change-Id: I6de9e11a605a53f206996a7b3acdd10849daa3a9
Reviewed-on: https://chromium-review.googlesource.com/1246830
Reviewed-by: Mattias Nissler <[email protected]>
Commit-Queue: Micah Morton <[email protected]>
Cr-Commit-Position: refs/heads/master@{#594727}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index e0453ed..30b2ace 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -2121,6 +2121,9 @@
 extern const char kUsbguardName[];
 extern const char kUsbguardDescription[];
 
+extern const char kShillSandboxingName[];
+extern const char kShillSandboxingDescription[];
+
 extern const char kUseMashName[];
 extern const char kUseMashDescription[];