Explain why the 'fileSystem' permission has no permission string

chrome/common/extensions/permissions/api_permission.cc

 // 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.
 
 #include "chrome/common/extensions/permissions/api_permission.h"
 
 #include "chrome/common/extensions/permissions/permissions_info.h"
 #include "grit/generated_resources.h"
 #include "ui/base/l10n/l10n_util.h"
 
@@ skipping 147 matching lines @@
 
     // Platform-app permissions.
     { kSocket, "socket", kFlagCannotBeOptional },
     { kAppWindow, "app.window" },
     { kAudioCapture, "audioCapture", kFlagNone,
       IDS_EXTENSION_PROMPT_WARNING_AUDIO_CAPTURE,
       PermissionMessage::kAudioCapture },
     { kVideoCapture, "videoCapture", kFlagNone,
       IDS_EXTENSION_PROMPT_WARNING_VIDEO_CAPTURE,
       PermissionMessage::kVideoCapture },
+    // "fileSystem" has no permission string because read-only access is only
+    // granted after the user has been shown a file chooser dialog and selected
+    // a file. Selecting the file is considered consent to read it.
     { kFileSystem, "fileSystem" },
     { kFileSystemWrite, "fileSystemWrite", kFlagNone,
       IDS_EXTENSION_PROMPT_WARNING_FILE_SYSTEM_WRITE,
       PermissionMessage::kFileSystemWrite },
     { kMediaGalleriesAllGalleries, "mediaGalleriesAllGalleries",
       kFlagCannotBeOptional,
       IDS_EXTENSION_PROMPT_WARNING_MEDIA_GALLERIES_ALL_GALLERIES,
       PermissionMessage::kMediaGalleriesAllGalleries },
   };
 
   for (size_t i = 0; i < ARRAYSIZE_UNSAFE(PermissionsToRegister); ++i) {
     const PermissionRegistration& pr = PermissionsToRegister[i];
     info->RegisterPermission(
         pr.id, pr.name, pr.l10n_message_id,
         pr.message_id ? pr.message_id : PermissionMessage::kNone,
         pr.flags);
   }
 
   // Register aliases.
   info->RegisterAlias("unlimitedStorage", kOldUnlimitedStoragePermission);
   info->RegisterAlias("tabs", kWindowsPermission);
   // TODO(mihaip): Should be removed for the M20 branch, see
   // http://crbug.com/120447 for more details.
   info->RegisterAlias("background", kTemporaryBackgroundAlias);
 }
 
 }  // namespace extensions