Update .gn to set a min SDK for macOS via GN.
Update .gn to set a min SDK for macOS via GN.

Currently, libyuv is setting this config via mac_sdk_min_build_override. The old
meechanism is deprecated, but cannot be removed until chromium is updated to no
longer require mac_sdk_min_build_override.

TBR=kjellander@chromium.org
Bug:chromium:740693
Change-Id: I71533c9ef20ac8d7584d50751ac5437da54e2cb5
Reviewed-on: https://chromium-review.googlesource.com/565636
Reviewed-by: Frank Barchard <52415189a3f67d375195cbbb6fdb1f1759498e57@google.com>

lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv