Async DNS: Disable on Linux until we parse nsswitch.conf

Async DNS: Disable on Linux until we parse nsswitch.conf

Right now, we don't examine nsswitch.conf, but we assume it has a
standard (hosts: files dns) configuration for hostname resolution.

We should check that that's true before activating the internal
resolver, and fall back to the OS resolver if it's not.

In the meantime, however, just disable Async DNS on Linux so it doesn't
break for users with nonstandard configurations.

BUG=117655
Committed: https://crrev.com/1a2a2738522cb00347f277e8e7d725a4e46ed053
Cr-Commit-Position: refs/heads/master@{#320160}

[Deprecated (see juliatuttle), 2015-03-10 22:23:43]

ttuttle@chromium.org changed reviewers:
+ davidben@chromium.org

[Deprecated (see juliatuttle), 2015-03-10 22:23:43]

ttuttle@chromium.org changed required reviewers:
+ davidben@chromium.org

[Deprecated (see juliatuttle), 2015-03-10 22:23:44]

PTAL, davidben.

[davidben, 2015-03-11 00:54:27]

lgtm

[Deprecated (see juliatuttle), 2015-03-11 15:44:21]

The CQ bit was checked by ttuttle@chromium.org

[Deprecated (see juliatuttle), 2015-03-11 15:44:23]

The CQ bit was unchecked by ttuttle@chromium.org

[Deprecated (see juliatuttle), 2015-03-11 20:06:18]

The CQ bit was checked by ttuttle@chromium.org

[commit-bot: I haz the power, 2015-03-11 20:06:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/990063004/1

[commit-bot: I haz the power, 2015-03-11 20:34:47]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-11 20:34:48]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[cbentzel, 2015-03-11 21:25:16]

cbentzel@chromium.org changed reviewers:
+ cbentzel@chromium.org

[cbentzel, 2015-03-11 21:25:16]

LGTM

[Deprecated (see juliatuttle), 2015-03-11 21:54:41]

The CQ bit was checked by ttuttle@chromium.org

[commit-bot: I haz the power, 2015-03-11 21:55:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/990063004/1

[commit-bot: I haz the power, 2015-03-11 22:06:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-11 22:07:13]

Patchset 1 (id:??) landed as
https://crrev.com/1a2a2738522cb00347f277e8e7d725a4e46ed053
Cr-Commit-Position: refs/heads/master@{#320160}