Fix dump_file_system

The change in r247631 broke the ability to compile the dump_file_system
tool.

This patch allows it to be built once more.

Tested with:

ninja -C out/Release dump_file_system
out/Release/dump_file_system <path-to-a-filesystem-dir>

BUG=
R=nhiroki@chromium.org, tzik@chromium.org

Review URL: https://codereview.chromium.org/153363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248769 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248769