Explain why the 'fileSystem' permission has no permission string

BUG=130452
TBR=mpcomplete@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10827088

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149159 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/permissions/api_permission.cc

@@ -165,6 +165,9 @@ void APIPermission::RegisterAllPermissions(
     { 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,