Enable <webview>.executeScript outside of Apps and Extensions [1]

extensions/common/host_id.h

 // Copyright 2015 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.
 
 #ifndef EXTENSIONS_COMMON_HOST_ID_H_
 #define EXTENSIONS_COMMON_HOST_ID_H_
 
 #include <string>
 
 // IDs of hosts who own user scripts.
 // A HostID is immutable after creation.
 struct HostID {
-  enum HostType { EXTENSIONS, WEBUI };
+  enum HostType { EXTENSIONS, WEBUI, HOST_TYPE_LAST = WEBUI };
 
   HostID();
   HostID(HostType type, const std::string& id);
   HostID(const HostID& host_id);
   ~HostID();
 
   bool operator<(const HostID& host_id) const;
   bool operator==(const HostID& host_id) const;
 
   HostType type() const { return type_; }
   const std::string& id() const { return id_; }
 
  private:
   // The type of the host.
   HostType type_;
 
   // Similar to extension_id, host_id is a unique indentifier for a host,
   // e.g., an Extension or WebUI.
   std::string id_;
 };
 
 #endif  // EXTENSIONS_COMMON_HOST_ID_H_