Migrate builder tryserver.chromium.mac:mac_chromium_variable_chrome to LUCI.

Buildbot: https://ci.chromium.org/buildbot/tryserver.chromium.mac/mac_chromium_variable_chrome
LUCI: https://ci.chromium.org/buildbucket/luci.chromium.try/LUCI%20mac_chromium_variable_chrome

I will be posting updates on changes of the migration status.
For the latest status, see
https://luci-migration.appspot.com/masters/tryserver.chromium.mac/builders/mac_chromium_variable_chrome