Revert "[signin] Remove signin.allowed pref."

This reverts commit 485addda95acf5a4d791fd419a195e7480cd29ff.

Reason for revert: Still needed for http://crbug.com/888776

Original change's description:
> [signin] Remove signin.allowed pref.
> 
> The signin.allowed policy has been deprecated. This CL removes both
> the signin.allowed policy and the signin.allowed preference that was
> used to back it up.
> 
> Bug: 870239
> 
> Change-Id: I91be7a30848ceffe31eabe3287b4672ae394a6ed
> Reviewed-on: https://chromium-review.googlesource.com/1225800
> Reviewed-by: Jochen Eisinger <[email protected]>
> Reviewed-by: Ramin Halavati <[email protected]>
> Reviewed-by: Julian Pastarmov <[email protected]>
> Reviewed-by: Marc Treib <[email protected]>
> Reviewed-by: Boris Sazonov <[email protected]>
> Commit-Queue: Mihai Sardarescu <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#593117}

[email protected],[email protected],[email protected],[email protected],[email protected],[email protected]

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 870239
Change-Id: I13f6471e8d70b8cd8224b1d0a94b86a324fbd4f4
Reviewed-on: https://chromium-review.googlesource.com/1245441
Commit-Queue: Mihai Sardarescu <[email protected]>
Reviewed-by: Mihai Sardarescu <[email protected]>
Reviewed-by: Boris Sazonov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#594269}
diff --git a/chrome/browser/supervised_user/supervised_user_constants.cc b/chrome/browser/supervised_user/supervised_user_constants.cc
index 63e4240..f6902f2 100644
--- a/chrome/browser/supervised_user/supervised_user_constants.cc
+++ b/chrome/browser/supervised_user/supervised_user_constants.cc
@@ -24,6 +24,7 @@
 const char kRecordHistoryIncludesSessionSync[] =
     "RecordHistoryIncludesSessionSync";
 const char kSafeSitesEnabled[] = "SafeSites";
+const char kSigninAllowed[] = "SigninAllowed";
 const char kUserName[] = "UserName";
 
 // NOTE: Do not change this value without changing the value of the