Revert "Revert "Async DNS: Remove toggle from about:flags""

I'm going to disable it on Linux entirely instead of restoring the flag.

I'll merge that disable to dev/beta/stable, then write a change that
actually examines nsswitch.conf to see if the hosts config is standard.

BUG=117655

Review URL: https://codereview.chromium.org/995833003

Cr-Commit-Position: refs/heads/master@{#320310}
3 files changed