Separate the syncing of extension settings and app settings into separate data

chrome/browser/sync/protocol/app_setting_specifics.proto

+// Copyright (c) 2011 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 an app setting.
+// This is the same as for an extension setting, but uses a separate datatype
+// in order to control syncability separately.
+
+// 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;
+
+import "sync.proto";
+import "extension_setting_specifics.proto";
+
+// Properties of app setting sync objects; just an extension setting.
+message AppSettingSpecifics {
+  optional ExtensionSettingSpecifics extension_setting = 1;
+}
+
+extend EntitySpecifics {
+  optional AppSettingSpecifics app_setting = 103656;
+}