Add a warning when developing an extension that uses old manifest version.

chrome/common/extensions/manifest.h

 // 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.
 
 #ifndef CHROME_COMMON_EXTENSIONS_MANIFEST_H_
 #define CHROME_COMMON_EXTENSIONS_MANIFEST_H_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 15 matching lines @@
 
   const std::string& extension_id() const { return extension_id_; }
   void set_extension_id(const std::string& id) { extension_id_ = id; }
 
   Extension::Location location() const { return location_; }
 
   // |error| will be non-empty if the manifest is malformed. |warnings| will
   // be populated if there are keys in the manifest that cannot be specified by
   // the extension type.
   void ValidateManifest(std::string* error,
-                        std::vector<std::string>* warnings) const;
+                        Extension::InstallWarningVector* warnings) const;
 
   // The version of this extension's manifest. We increase the manifest
   // version when making breaking changes to the extension system. If the
   // manifest contains no explicit manifest version, this returns the current
   // system default.
   int GetManifestVersion() const;
 
   // Returns the manifest type.
   Extension::Type type() const { return type_; }
 
@@ skipping 44 matching lines @@
   scoped_ptr<base::DictionaryValue> value_;
 
   Extension::Type type_;
 
   DISALLOW_COPY_AND_ASSIGN(Manifest);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_COMMON_EXTENSIONS_MANIFEST_H_