Add Sync datatype for shared managed user settings.

sync/protocol/managed_user_shared_setting_specifics.proto

+// Copyright (c) 2013 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 managed user shared settings.
+
+// Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
+// any fields in this file.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+option retain_unknown_fields = true;
+
+package sync_pb;
+
+// Properties of managed user shared setting sync objects.
+message ManagedUserSharedSettingSpecifics {
+  // The MU ID for the managed user to whom the setting applies.
+  optional string mu_id = 1;
+  // The key of the setting.
+  optional string key = 2;
+  // The setting value. The setting is a JSON encoding of an arbitrary
+  // Javascript value.
+  optional string value = 3;
+  // This flag is set by the server to acknowledge that it has committed a
+  // change to a setting.
+  optional bool acknowledged = 4 [default = false];
+}