Migrate builder tryserver.chromium.linux:chromeos_x86-generic_chromium_compile_only_ng to LUCI.

Buildbot: https://ci.chromium.org/buildbot/tryserver.chromium.linux/chromeos_x86-generic_chromium_compile_only_ng
LUCI: https://ci.chromium.org/buildbucket/luci.chromium.try/LUCI%20chromeos_x86-generic_chromium_compile_only_ng

I will be posting updates on changes of the migration status.
For the latest status, see
https://luci-migration.appspot.com/masters/tryserver.chromium.linux/builders/chromeos_x86-generic_chromium_compile_only_ng