Install CIPD ninja using DEPS

See chromium/src's CL https://crrev.com/c/3869740 for the context.

Bug: chromium:1340825
Change-Id: I4429bc7a2a1e57f559c4c958234ce150842448df
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/104340
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/.gitignore b/.gitignore
index a66733d..6fb333a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -22,6 +22,7 @@
 /third_party/jsoncpp
 /third_party/llvm-build
 /third_party/markupsafe
+/third_party/ninja
 /third_party/node
 /third_party/node-addon-api
 /third_party/node-api-headers