Install ninja using DEPS

We are going to install ninja CIPD to third_party.
https://crrev.com/c/4005475

This CL adds `.gitignore` and `README.nacl` for the change.

Bug: chromium:1340825
Change-Id: Ibf6610432d48de849a9eb886ba25fbfb29dd63d0
Reviewed-on: https://chromium-review.googlesource.com/c/native_client/src/third_party/+/4005416
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Auto-Submit: Junji Watanabe <jwata@google.com>