This adds a minimal mb config to switch the v8 staging bot
to mb. External mb documentation:
https://chromium.googlesource.com/chromium/src/tools/mb/+/HEAD/docs/user_guide.md

After this is tested on the staging bot, mb configs for the
remaining bots will be provided and all bots will be
systematically switched.

BUG=chromium:616035
NOTRY=true

Review-Url: https://codereview.chromium.org/2024893002
Cr-Commit-Position: refs/heads/master@{#36847}