Add mac_xcode_version gclient arg to DEPS Chromium's //build/ requires the DEPS file to produce gclient_args.gni that contains a mac_xcode_version variable Bug: angleproject:4878 Change-Id: I24421173347e1b508913d49793a065ccd52a47e7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2321395 Commit-Queue: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>