Fix filesystem API file_handlers to work for drive on ChromeOS.

webkit/fileapi/file_system_mount_point_provider.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 WEBKIT_FILEAPI_FILE_SYSTEM_MOUNT_POINT_PROVIDER_H_
 #define WEBKIT_FILEAPI_FILE_SYSTEM_MOUNT_POINT_PROVIDER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 131 matching lines @@
   // Grants access to |virtual_path| from |origin_url|.
   virtual void GrantFileAccessToExtension(
       const std::string& extension_id,
       const base::FilePath& virtual_path) = 0;
   // Revokes file access from extension identified with |extension_id|.
   virtual void RevokeAccessForExtension(
         const std::string& extension_id) = 0;
   // Gets virtual path by known filesystem path. Returns false when filesystem
   // path is not exposed by this provider.
   virtual bool GetVirtualPath(const base::FilePath& file_system_path,
-                              base::FilePath* virtual_path) = 0;
+                              base::FilePath* virtual_path) const = 0;
 };
 
 }  // namespace fileapi
 
 #endif  // WEBKIT_FILEAPI_FILE_SYSTEM_MOUNT_POINT_PROVIDER_H_