Let git ignore the third_party dir to avoid unnecessary clean-ups.
Let git ignore the third_party dir to avoid unnecessary clean-ups.

BUG=chromium:433273
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/728983004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25365}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh