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: refs/heads/master@{#19006}
diff --git a/.gn b/.gn
index 4046d94..0079aaa 100644
--- a/.gn
+++ b/.gn
@@ -36,4 +36,6 @@
   # the args.gn file but this setting will be ignored because we don't support
   # component builds.
   is_component_build = false
+
+  mac_sdk_min = "10.11"
 }