FWIW, I have 1.12.0 building locally, and shill builds cleanly against it.

I intend to wait for 1.12.0 to become available in gentoo though so we can inherit their ebuild.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/portage-stable/+/62134e0daf260a1da7ebc4cb9a5571c9ee3ed32a

commit 62134e0daf260a1da7ebc4cb9a5571c9ee3ed32a
Author: Mattias Nissler <mnissler@chromium.org>
Date: Mon Oct 24 13:02:55 2016

Update net-dns/c-ares to version 1.12.

This upgrades c-ares to version 1.12, which contains the official fix
for the mkquery heap overflow. This is based on gentoo's
c-ares-1.12.0.ebuild, but with changes to make it work with EAPI=5.

BUG= chromium:651760 
TEST=c-ares and its reverse dependencies build successfully on cyan.
CQ-DEPEND=CL:402328

Change-Id: I733aab207e9107564260b51818c683a7e5aa25fa
Reviewed-on: https://chromium-review.googlesource.com/400822
Commit-Ready: Mattias Nissler <mnissler@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[delete] https://crrev.com/ecb204763ab0a44ac4f3394bc3d765f59e732ca9/net-dns/c-ares/c-ares-1.7.5-r1.ebuild
[add] https://crrev.com/62134e0daf260a1da7ebc4cb9a5571c9ee3ed32a/net-dns/c-ares/files/c-ares-1.12.0-remove-tests.patch
[add] https://crrev.com/62134e0daf260a1da7ebc4cb9a5571c9ee3ed32a/net-dns/c-ares/c-ares-1.12.0.ebuild
[delete] https://crrev.com/ecb204763ab0a44ac4f3394bc3d765f59e732ca9/net-dns/c-ares/c-ares-1.7.5.ebuild
[delete] https://crrev.com/ecb204763ab0a44ac4f3394bc3d765f59e732ca9/net-dns/c-ares/c-ares-1.7.4.ebuild
[modify] https://crrev.com/62134e0daf260a1da7ebc4cb9a5571c9ee3ed32a/net-dns/c-ares/Manifest

c-ares upgraded to 1.12.0 package on Cyan  M56-8950.0.0. closing this as verified.