ios: Simplify -stdlib=libc++ logic.

ios: Simplify -stdlib=libc++ logic.

We don't support building on 10.6 or with the 10.6 SDK any more, so it should
be fine to always use libc++ unconditionally for iOS builds.

No intended behavior change.

BUG=366744, 402921
Committed: https://crrev.com/72fb7d90b3f09b537f118d6be4d2998f392e11a4
Cr-Commit-Position: refs/heads/master@{#348852}

[Nico, 2015-09-14 21:52:07]

thakis@chromium.org changed reviewers:
+ sdefresne@chromium.org

[sdefresne, 2015-09-15 08:50:55]

The CQ bit was checked by sdefresne@chromium.org

[sdefresne, 2015-09-15 08:50:56]

lgtm

[commit-bot: I haz the power, 2015-09-15 08:51:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1334273004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1334273004/1

[commit-bot: I haz the power, 2015-09-15 08:57:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-15 08:57:47]

Patchset 1 (id:??) landed as
https://crrev.com/72fb7d90b3f09b537f118d6be4d2998f392e11a4
Cr-Commit-Position: refs/heads/master@{#348852}

[commit-bot: I haz the power, 2015-09-23 12:43:23]

Patchset 1 (id:??) landed as
https://crrev.com/72fb7d90b3f09b537f118d6be4d2998f392e11a4
Cr-Commit-Position: refs/heads/master@{#348852}