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 <fbarchard@google.com>

.gn

@@ -47,3 +47,6 @@ exec_script_whitelist = [
   "//build/toolchain/win/BUILD.gn",
 ]
 
+default_args = {
+  mac_sdk_min = "10.11"
+}