Add MB config for linux_chromium_chromeos_rel_ng.
Add MB config for linux_chromium_chromeos_rel_ng.

We missed the tryserver config when adding the CI config.

TBR=aneeshm@chromium.org
BUG=481692

Review URL: https://codereview.chromium.org/1846013002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384400}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk