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

This reverts commit eabf1f5baba38d46921acd6edda594f942f7d6a1.

Removing the toggle in about:flags breaks a workaround for users who
have nonstandard nsswitch configurations for hosts.

Revert the removal until we properly check nsswitch.conf before
assuming a standard configuration.

BUG=117655

Review URL: https://codereview.chromium.org/987133003

Cr-Commit-Position: refs/heads/master@{#319725}
3 files changed