GitLab

Will use mb for gn args of chromium_codesearch.py. Add gn configs in
mb_config.pyl
(https://chromium.googlesource.com/chromium/tools/build/+/master/scripts/slave/recipes/chromium_codesearch.py#49)

BUG=718658

Review-Url: https://codereview.chromium.org/2865593002
Cr-Commit-Position: refs/heads/master@{#469577}