Metal: Avoid leaking MTLDevice lists in DisplayMtl Hold the lists via ObjCObj<>. Patch by David Kilzer <ddkilzer@apple.com> Bug: angleproject:6920 Change-Id: Ia25cd59b0dd2e81367d8bf5ca50a22ca02547940 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3568381 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>