Make web_drag_source.cc compile on Posix.

chrome/common/temp_scaffolding_stubs.h

 // Copyright (c) 2009 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_TEMP_SCAFFOLDING_STUBS_H_
 #define CHROME_COMMON_TEMP_SCAFFOLDING_STUBS_H_
 
 // This file provides declarations and stub definitions for classes we encouter
 // during the porting effort. It is not meant to be permanent, and classes will
 // be removed from here as they are fleshed out more completely.
@@ skipping 721 matching lines @@
     return GURL();
   }
 };
 
 class OSExchangeData {
  public:
   void SetString(const std::wstring& data) { NOTIMPLEMENTED(); }
   void SetURL(const GURL& url, const std::wstring& title) { NOTIMPLEMENTED(); }
 };
 
+class BaseDragSource {
+};
+
 #endif  // CHROME_COMMON_TEMP_SCAFFOLDING_STUBS_H_