Add empty file to prevent git clean from deleting the tools folder. Bug: angleproject:1569 Change-Id: Ic0161260e1c19fb660be06cd5a844f9031a77ef0 Reviewed-on: https://chromium-review.googlesource.com/926863 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>