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

chrome/browser/chromeos/file_manager/volume_manager.h

 // Copyright 2013 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_MANAGER_VOLUME_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_FILE_MANAGER_VOLUME_MANAGER_H_
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 46 matching lines @@
   NUM_VOLUME_TYPE,
 };
 
 struct VolumeInfo {
   VolumeInfo();
   ~VolumeInfo();
 
   // The ID of the volume.
   std::string volume_id;
 
-  // The ID for provided file systems. If other type, then equal to zero.
-  int file_system_id;
+  // The ID for provided file systems. If other type, then empty string. Unique
+  // per providing extension.
+  std::string file_system_id;
 
   // The ID of an extension providing the file system. If other type, then equal
   // to an empty string.
   std::string extension_id;
 
   // The type of mounted volume.
   VolumeType type;
 
   // The type of device. (e.g. USB, SD card, DVD etc.)
   chromeos::DeviceType device_type;
@@ skipping 145 matching lines @@
 
   // Note: This should remain the last member so it'll be destroyed and
   // invalidate its weak pointers before any other members are destroyed.
   base::WeakPtrFactory<VolumeManager> weak_ptr_factory_;
   DISALLOW_COPY_AND_ASSIGN(VolumeManager);
 };
 
 }  // namespace file_manager
 
 #endif  // CHROME_BROWSER_CHROMEOS_FILE_MANAGER_VOLUME_MANAGER_H_