Apps&Extensions for supervised users: Add Extension::WAS_INSTALLED_BY_CUSTODIAN flag and proto entry

sync/protocol/extension_specifics.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Sync protocol datatype extension for extensions.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 21 matching lines @@
   optional bool enabled = 4;
   // Whether or not this extension is enabled in incognito mode.
   optional bool incognito_enabled = 5;
 
   // The name of the extension. Used for bookmark apps.
   optional string name = 6;
 
   // Whether this extension was installed remotely, and hasn't been approved by
   // a user in chrome yet.
   optional bool remote_install = 7;
+
+  // Whether this extension was installed by the custodian of a supervised user.
+  optional bool installed_by_custodian = 8;
 }