Renderer11: expose a metric for the Buffer11 CPU copies size This will allow us to measure the impact of subsequent optimizations on the memory consumption of Chrome. BUG=angleproject:916 Change-Id: Ib57fdfcc2950917eff33080bfb3db574595ecb6d Reviewed-on: https://chromium-review.googlesource.com/284637 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>