Add empty list indicator for Physical Web WebUI

components/physical_web/webui/physical_web_ui_constants.cc

 // Copyright 2016 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.
 
 #include "components/physical_web/webui/physical_web_ui_constants.h"
 
 namespace physical_web_ui {
 
 // Resource paths.
 const char kPhysicalWebJS[] = "physical_web.js";
 const char kPhysicalWebCSS[] = "physical_web.css";
 
 // Message handlers.
 const char kRequestNearbyUrls[] = "requestNearbyURLs";
 const char kPhysicalWebItemClicked[] = "physicalWebItemClicked";
 
 // Strings.
 const char kTitle[] = "title";
+const char kEmptyMessage[] = "emptyMessage";
 const char kPageInfoDescription[] = "pageInfoDescription";
 const char kPageInfoIcon[] = "pageInfoIcon";
 const char kPageInfoTitle[] = "pageInfoTitle";
 const char kResolvedUrl[] = "resolvedUrl";
 const char kIndex[] = "index";
 const char kReturnNearbyUrls[] = "returnNearbyURLs";
 const char kMetadata[] = "metadata";
 
 }  // namespace physical_web_ui