Update zero length for debug marker commands If the marker length passed to InsertEventMarkerEXT or PushGroupMarkerEXT commands is zero, compute the actual marker length in the Context so that the backends could always rely on the passed length. Bug: angleproject:408709155 Bug: angleproject:417757002 Change-Id: Ifa36d8d16c7c808855a7500556b569587cdc89a1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6547538 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>