Use libc++ by default on iOS

Use libc++ by default on iOS

libc++ is the recommended C++ library for iOS development so use
it by default when targeting iOS.

clang ToT ships with all the required header and libraries to
support compiling with -stdlib=libc++ so remove the exception.

BUG=366744
Committed: https://crrev.com/7da3d8874ba759574edf778c58de024fe0974925
Cr-Commit-Position: refs/heads/master@{#295715}

[sdefresne, 2014-09-19 13:37:11]

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

[sdefresne, 2014-09-19 13:37:12]

Please have a look.

[rohitrao (ping after 24h), 2014-09-19 13:41:10]

LGTM

[sdefresne, 2014-09-19 15:15:27]

The CQ bit was checked by sdefresne@chromium.org

[commit-bot: I haz the power, 2014-09-19 15:16:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/589513003/1

[commit-bot: I haz the power, 2014-09-19 15:59:31]

Committed patchset #1 (id:1) as d3158a160587273002a8a92945fe05af412eb545

[commit-bot: I haz the power, 2014-09-19 16:00:00]

Patchset 1 (id:??) landed as
https://crrev.com/7da3d8874ba759574edf778c58de024fe0974925
Cr-Commit-Position: refs/heads/master@{#295715}