Migrate builder tryserver.chromium.linux:chromeos_amd64-generic_variable to LUCI.

Buildbot: https://ci.chromium.org/buildbot/tryserver.chromium.linux/chromeos_amd64-generic_variable
LUCI: https://ci.chromium.org/buildbucket/luci.chromium.try/LUCI%20chromeos_amd64-generic_variable

I will be posting updates on changes of the migration status.
For the latest status, see
https://luci-migration-dev.appspot.com/masters/tryserver.chromium.linux/builders/chromeos_amd64-generic_variable