[fsp] Let extensions decide about the file system id.

chrome/browser/chromeos/file_system_provider/provided_file_system_info.h

 // Copyright 2014 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_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_PROVIDED_FILE_SYSTEM_INFO_H_
 #define CHROME_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_PROVIDED_FILE_SYSTEM_INFO_H_
 
 #include <string>
 
 #include "base/files/file_path.h"
 
 namespace chromeos {
 namespace file_system_provider {
 
 // Contains information about the provided file system instance.
 class ProvidedFileSystemInfo {
  public:
   ProvidedFileSystemInfo();
   ProvidedFileSystemInfo(const std::string& extension_id,
-                         int file_system_id,
+                         const std::string& file_system_id,
                          const std::string& file_system_name,
                          const base::FilePath& mount_path);
 
   ~ProvidedFileSystemInfo();
 
   const std::string& extension_id() const { return extension_id_; }
-  int file_system_id() const { return file_system_id_; }
+  const std::string& file_system_id() const { return file_system_id_; }
   const std::string& file_system_name() const { return file_system_name_; }
   const base::FilePath& mount_path() const { return mount_path_; }
 
  private:
   // ID of the extension providing this file system.
   std::string extension_id_;
 
-  // ID of the file system, used internally.
-  int file_system_id_;
+  // ID of the file system.
+  std::string file_system_id_;
 
   // Name of the file system, can be rendered in the UI.
   std::string file_system_name_;
 
   // Mount path of the underlying file system.
   base::FilePath mount_path_;
 };
 
 }  // namespace file_system_provider
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_FILE_SYSTEM_PROVIDER_PROVIDED_FILE_SYSTEM_INFO_H_