Metal: Avoid using retainAssign() Use more native C++ name for assign: operator=. Bug: angleproject:393263506 Change-Id: Ia31a79fbb15474ba754034338340ef5c3ed6e8ec Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6216871 Reviewed-by: Geoff Lang <geofflang@chromium.org> Auto-Submit: Kimmo Kinnunen <kkinnunen@apple.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>