Metal: Use -weak_framework to support OS 10.10. This OS is still tested in Chromium configurations. e.g.: https://ci.chromium.org/p/chromium/builders/ci/Mac10.10%20Tests/47668 Bug: chromium:1014228 Bug: angleproject:2634 Change-Id: Iadefd461ba8bbaec03dcc711227580f6ecdcad6a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1860105 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>