Migrate builder tryserver.chromium.linux:cast_shell_linux to LUCI.

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

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/cast_shell_linux