[infra] Add fetch-ninja script

Allows folks who do not use gclient to still get use ninja
after https://groups.google.com/a/google.com/g/chromium-dev-internal/c/Q9zzGPT8bMc

depot_tools appears to look for ninja in third_party/ninja,
so that's where we put it.

Change-Id: I1c80218f440324504cc485af47dcca287a19b509
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/607062
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
2 files changed