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

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

BUG=chromium:740693
TBR=kjellander@webrtc.org

Review-Url: https://codereview.webrtc.org/2979603002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#19006}

TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc