Migrate builder tryserver.chromium.win:win8_chromium_gn_upload to LUCI.

Buildbot: https://ci.chromium.org/buildbot/tryserver.chromium.win/win8_chromium_gn_upload
LUCI: https://ci.chromium.org/buildbucket/luci.chromium.try/LUCI%20win8_chromium_gn_upload

I will be posting updates on changes of the migration status.
For the latest status, see
https://luci-migration.appspot.com/masters/tryserver.chromium.win/builders/win8_chromium_gn_upload