Revert of base: Remove using:: declaration from version.h header.

base/version.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 BASE_VERSION_H_
 #define BASE_VERSION_H_
 
 #include <stdint.h>
 #include <string>
 #include <vector>
 
 #include "base/base_export.h"
+#include "base/basictypes.h"
 
 namespace base {
 
 // Version represents a dotted version number, like "1.2.3.4", supporting
 // parsing and comparison.
 class BASE_EXPORT Version {
  public:
   // The only thing you can legally do to a default constructed
   // Version object is assign to it.
   Version();
@@ skipping 35 matching lines @@
   const std::string GetString() const;
 
   const std::vector<uint32_t>& components() const { return components_; }
 
  private:
   std::vector<uint32_t> components_;
 };
 
 }  // namespace base
 
+// TODO(xhwang) remove this when all users are updated to explicitly use the
+// namespace
+using base::Version;
+
 #endif  // BASE_VERSION_H_