testing/variations/fieldtrial_testing_config.json does not currently allow the feature
to specify low-end devices even when the feature is released only for low-end. 
This causes a problem where a high-end perf bot complains about the memory regression
introduced by the feature, though the feature does not run on high-end in the wild.

Thus we would like to add "low-end" device specification for fieldtrial_testing_config.

See the group discussion here: https://groups.google.com/a/google.com/forum/?utm_medium=email&utm_source=footer#!msg/chrome-metrics-team/NvAXA-bZuUU/cKzwD4aeBgAJ