Metal: ProvokingVertexHelper stop using a separate cmdbuffer This simplifies the code. Without this you always need to be aware that the provoking vertex helper's command buffer might have commands in it that need to be committed before starting other operations. Bug: angleproject:7643 Change-Id: I1c73d451c12c268ecbdb6a1cd63a77c291949ef9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3884275 Commit-Queue: Gregg Tavares <gman@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Kyle Piddington <kpiddington@apple.com>