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:

Review URL: https://codereview.chromium.org/1437173002