Metal: Implement InsertEventMarker * Added a missing command buffer call. * Renamed related symbols to match Metal API. * Removed ConvertMarkerToString helper because marker length is now handled in the frontend. Bug: angleproject:417857731 Change-Id: Ifa9edaafaf16af9913420554e5847a5a25b8d2f1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6549282 Reviewed-by: Quyen Le <lehoangquyen@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>