Fix Buffer11 staging buffer revision update. This bug could trigger a potential extra copy when drawing with a pack buffer (or possibly packing vertex/index data). BUG=angle:912 Change-Id: Ifead6524322aac5bc0be79a05f4db08c8f5a3225 Reviewed-on: https://chromium-review.googlesource.com/249181 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>