Target Mac bots at 10.7+libc++ to approximate Chrome's 10.6+libc++.

Target Mac bots at 10.7+libc++ to approximate Chrome's 10.6+libc++.

This is necessary if we want to start using C++11 library features.

Chrome's got their own custom libc++-for-10.6.
This is a somewhat lazy approximation of that.

Chrome should still guard us from using 10.7-specific features
until 10.6-10.8 all go away.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/d7b25c734e6835152451fbe2ff3b809c5006ad51

[mtklein, 2015-11-12 15:42:29]

mtklein@google.com changed reviewers:
+ bungeman@google.com, mtklein@google.com

[mtklein_C, 2015-11-12 15:42:46]

mtklein@chromium.org changed reviewers:
- mtklein@google.com

[mtklein_C, 2015-11-12 15:42:46]

Er, one more time.

[mtklein_C, 2015-11-12 15:42:48]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-11-12 15:42:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1437173002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1437173002/1

[commit-bot: I haz the power, 2015-11-12 15:43:00]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-11-12 21:42 UTC

[bungeman-skia, 2015-11-12 15:54:47]

If we're going to do this, let's also remove the precursor to this from
gyp/tools#skdiff at the same time.

[mtklein_C, 2015-11-12 16:30:34]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-11-12 16:30:40]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1437173002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1437173002/20001

[bungeman-skia, 2015-11-12 16:31:17]

lgtm

[mtklein_C, 2015-11-12 16:32:30]

The CQ bit was unchecked by mtklein@chromium.org

[mtklein_C, 2015-11-12 16:32:34]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-11-12 16:32:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1437173002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1437173002/20001

[commit-bot: I haz the power, 2015-11-12 16:41:44]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/d7b25c734e6835152451fbe2ff3b809c5006ad51